Seminar food guidelines

The food should be easy to eat from a plastic plate in one’s lap, without paying attention to it. It should not require a knife, fork, spoon or chopsticks. Sandwiches fulfill these criteria. Sushi can also be eaten with one’s fingers. Sandwiches should not be so thick that they have to be disassembled to fit in the mouth. Sandwiches should not contain ingredients that are difficult to bite through, for example prosciutto, non-crispy bacon, meat with tendons in it.
The food should not drip or stain the hands, especially with a greasy or otherwise difficult-to-remove sauce. Wraps should not have the bottom cut off or contain a thin sauce that leaks through the bottom. Sandwiches should not have contents falling out – avoid a thick stack of many fillings between the breads. A single filling can be thicker, e.g. a chicken breast. Biting into the food should not cause the food to fall apart (rice paper rolls have this problem) or something to squirt out the other end (as happens with sandwiches with a lot of sauce or mayonnaise).
Avoid ingredients with a strong, specific taste that some people love and some hate. Examples are sauerkraut, pickles, olives, capers, kimchi, herring, anchovies, hot spices. The food should be like a politician – trying to please everyone, avoiding controversy. Spices and sauces can accompany the food separately, like wasabi and soy sauce with sushi – then everyone can add the amount they like.
The mechanics of eating the food is as important as the taste. The ease of eating of various forms of food can be tested in a seminar-like situation: eating sitting, with a small plastic plate in one’s lap, no table, only occasionally glancing at the food.

The obsolete PhD degree

Let’s distinguish the knowledge from the degree first. The average skill requirement of jobs (measured in years of education for example) is rising over time, so people need more knowledge before entering the labour market. What is obsolete is the packaging of that knowledge into degrees and perhaps its teaching in universities.
The PhD takes six years on average ( and during that time the student is guided by one or at most a few advisors. Working on the same topic on years is often necessary to become an expert, so unavoidable. But being tied to the same advisor is a throwback to the medieval guild system where the apprentice and journeyman work years for the master. It means seeing only one viewpoint or set of techniques. Most importantly, the topic of the thesis is limited to what the advisor is competent in (sometimes a laissez-faire advisor allows a dissertation on an unfamiliar subject, which is even worse – incompetent advising follows). Taking courses from other faculty in the same department or university broadens the horizons a bit, but there may be an institutional culture that introduces biases, or expertise in some fields may simply be missing from the university. Attending conferences again broadens the mind, but conferences are few and far between. Suggestions that run counter to the advisor’s views may be interpreted as wrong by a novice graduate student.
Ideally, a trainee researcher would be advised by the whole world’s scientific community, mostly but not exclusively by people in the same discipline. Electronic communication makes this easy. Many different viewpoints would be explained to the graduate student, interpersonal issues would be easier to resolve by changing advisors (no lock-in to one person who determines one’s career prospects). People who just use students as free labour without providing much in return would suddenly become lonely. The problem is moral hazard – if no specific person has responsibility for a student, indefinite postponement of advising effort may occur. Credit for useful advice would be spread between many people, which dilutes incentives. In short, advising is a public good.
Still, public goods are sometimes provided, despite the difficulty of explaining this with a rational agent model. People write free software, answer questions from strangers in forums, upload advice and instructions on many topics. This suggests some volunteer advisors would step forward under a shared responsibility system. The advisor pool may become more ideologically biased than now, because people who want to spread propaganda on their strong views have a greater incentive to volunteer advice. They do this on the internet, after all. Similar incentives for shrill prophets operate in universities, but if each faculty member is required to advise some students or if there is a cap on how many disciples one can take, there is less scope for indoctrinating the masses. Such restrictions can be imposed online to some extent. There could be a reputation mechanism among the advisors, so the crackpots are labelled as such. The larger pool of opinions may balance the biases.
The economies of scale in advising one student are reduced with sharing. A single advisor per student means that during most of the PhD program, the advisor is already familiar with the student’s work and only needs to read the new part each week. With many advisors, each would need to devote time to the same material. Some sharing of responsibilities (one reads the introduction, another the conclusion) is possible, but the interdependence of the parts of the research does not permit full splitting.
Another medieval aspect of the PhD is paying for the received teaching in labour, not money. Graduate students may be free from tuition and may even get a scholarship, but in return have to work as teaching assistants or do the advisor’s research in their lab. Less ethical help also occurs, such as reviewing papers the advisor is officially the referee of. Inefficiencies of a barter economy are introduced. Instead of paying for the program with money earned in the job the student is the most productive or happy in, the student is forced to work as a teaching assistant and essentially pay the difference between a fair market wage and the teaching assistant wage to the university. Further, the teaching work is restricted to the university of the PhD program, even if other universities need teachers more and offer higher wages. This gives the university market power and allows it to depress grad student salaries.
A doctoral program may lose money directly, in the sense that teaching the grad students is more expensive (due to small classes, advanced material, so more professor time per student) than their TA work. The fact that universities still keep the PhD programs suggests the existence of indirect benefits. One is reputation – attracting paying undergraduate and Master’s students. In some countries, an institution is not allowed to call itself a university if it does not teach at the doctoral level. Altruism by the higher education sector is possible, even if John Quiggin’s quote “never stand between a Vice-Chancellor and a bucket of money” suggests otherwise.
One utopic proposal is an online system where graduate students and advisors sign up and can talk over video calls, send emails etc. It keeps a record who communicated with whom and how much. Later, data on the academic achievement and job market performance of students can be added, so advisors can be rewarded for their students’ success. There may also be some popularity index, meaning students rate their advisors and vice versa. But in the end, an advisor’s contribution should matter more than popularity, so the latter is optional. Advisors may look at and rate each other’s advising sessions to limit the spread of bad advice. Students can collaborate and may decide to meet in person.
For experimental science, lab space can be rented by student cooperatives. Instruction in the use of equipment can be given via video. Classroom space can also be rented directly by groups of students if needed. The students may pay advisors. Some people may only advise conditional on payment. Students may teach other students (including TA jobs), whether for money or pro bono. The system would cut out the middlemen – university administrators – making education cheaper for society. Of course, in the lab and classroom rental business, other middlemen would appear and take their share.

Assumptions in communication

All communication relies on assumptions, for example about the meanings of words. If someone says: „Pass the salt, please,” then without assumptions, endless clarifications would be needed about the meaning of „pass” and „salt”, about the language used and whether the request is a joke or carries some hidden meaning. If the requester clarifies: „I did not mean it as a joke, I really want you to give me the salt,” then clarifications about the clarification would result. Again, the clarification may be a joke (we shouldn’t assume it is not) or use words like „not” in an ironic way with meaning the opposite of the usual (we shouldn’t assume the usual meaning). There would be an endless cycle of clarifying the clarifications of… of clarifications.

Real vs movie fighting

It will surprise nobody that real fighting or full-contact competition differs from movie fighting. What is perhaps less obvious is that the incentives for the actions are completely opposite. The actions themselves are not completely opposite, because movie fighting is supposed to look somewhat like a real fight, which constrains the difference between them.
An obvious incentive difference is the desire to hurt an opponent in a real fight vs not hurt a fellow actor. A more subtle distinction is that in a real fight or competition, nobody wants the opponent to see a punch or kick coming. In a movie, the flashier and more visible the attack and defense, the better. So in a real fight or competition, the movements are quick, preferably without wind-up by other parts of the body, mostly in a direct line from one body to the other (although some curved punches and kicks are used). The movements may be masked by feints, but these are subtle, like eyes flicking right while punching with the left. In a movie, the kicks especially move in long visible arcs, with the body turning 360 degrees in some cases, and not too fast. Every move is designed to be seen by the audience, which implies seen by the opponent.
In a movie fight, the techniques should not repeat, otherwise the audience gets bored. In a real fight, the only reason not to use one’s best move exclusively is the need to surprise the opponent. Only a few of the most effective techniques are used. Another reason for this is that real fights end quickly (not counting the posturing and shouting), so there is not much time to showcase a variety of punches and kicks. A wider range of moves is used in competitions, but still not close to the range in movie fights.


Bullshido is a term used for martial arts that make false claims about themselves, for example that the art helps a much smaller person defeat a larger one, or defend against a knife barehanded, or defeat multiple opponents. Such claims are marketing, but believing them puts people in danger, because instead of screaming and running, the believers may try to fight against hopeless odds and get injured. There are websites dedicated to exposing and ridiculing bullshido.
In addition, one should distinguish fighting and self-defense, and thus also unarmed combat training from self-defense classes. Detailed explanations are e.g. on the website Briefly, self-defense is about (1) avoiding crime (specific times, places, people), (2) noticing dangerous situations developing, (3) escaping them, (4) negotiating if escape is impossible (giving away your wallet and phone to avoid a beating), and (5) only as a last resort, fighting. Just like a castle has multiple concentric walls, self-defense has multiple concentric layers. Just like war happens when diplomacy and sanctions have failed, fighting happens when all other layers of self-defense have failed.
Returning to bullshido, one should apply the general principle that the burden of proof is on the maker of a claim. Some claims are hard to test without substantial danger, for example defending against a knife barehanded. But fighting larger people or many of them can be done with reasonable safety using boxing gloves, shin pads, soft helmets and similar protective equipment. The test should be scientifically rigorous, not like the demonstrations of martial arts clubs where the attackers and defenders have agreed on the moves beforehand. Such demonstrations are just theatre. In a real test, the attackers and defenders should not know each other, should not have the opportunity to collude, and each should be motivated to succeed, e.g. by a significant monetary prize for the winner. All moves should be made with maximal speed, without unrealistic „winding-up” movements calling attention to when and where the punch or kick will come.
Tests of this sort have been conducted for bare-handed fighting, namely in the Ultimate Fighting Championship (UFC) in the 1990s. There were several matches between traditional martial artists (karate, sumo) and more modern martial artists (Brazilian jiu jitsu, kickboxing). Traditional martial arts did not fare well. The evolutionary process in UFC and similar competitions, e.g. Pride, has led to the development of mixed martial arts (MMA) that is actually effective in bare-handed one-on-one fighting.
In general, martial arts that permit a wide range of full-speed, full-strength moves in competitions encourage more realistic training and improve fighting ability. Some traditional martial arts are included in this category, e.g. judo, wrestling.
Knife-defense can be tested with some realism using imitation knives. Even a rubber knife or marker pen can penetrate an eye or throat, so when using these, safety goggles and neck protection should be worn. A better fake knife is made of foam rubber and lightly coated in paint. The paint makes it very obvious when the knife has connected with the body. Without such an objective marking of a „cut”, the bullshido artist subject to the test can claim that the fake knife did not connect with them. A shortcoming of such a test is that the knife usually connects with a hand or forearm first, in which case many types of cuts from a real knife would make the arm useless and let the attacker get to the torso. Examples are cutting a nerve, tendon or muscle needed to move the arm. A safe fake knife does not make a hand useless, so the defender can continue using it in defense. There should be a referee competent to evaluate the damage to the hand or other part of the body that would have resulted from the „cut” (paint stripe) that the fake knife made. The referee should order the defender not to use a limb if that limb would be useless had the cut been real. One way to make a fake knife more realistic, but slightly more dangerous, is to add an electroshock ability to the fake knife. This can make a contacted limb go numb and become less useful for defense.
The idea of the knife measuring contact in some way has been used in fencing. The foils used sometimes have a pressure sensor in the tip that detects a touch on the opponent.
Training with an imitation knife and a motivated opponent who does not collaborate in his or her defeat will quickly cure the illusion of being able to defend against a knife barehanded. The same applies to most weapons, e.g. sticks, chains.
I have two stories about my experience with bullshido. In my early teens I took karate classes for a couple of years. Among other things, the classes taught how to twist an opponent’s arm or hand in various ways (and catching their punching hand prior to doing so). In training, I managed quite well to catch a punch and put the opponent on the ground with an arm twist. This is because the training was theatre and the opponent collaborated.
Upon hearing that I had learned martial arts, an adult construction worker offered to let me practise on him in the following way. He held his hand steady in front of him and I could try to twist it in any way I liked, using two hands against his one. He did not interfere with me in any way, just held the hand steady against any force. He was a physically average man, not some weightlifter or giant. I was about 15 years old, above average in size (about his height) and in good physical shape, but I could not twist his arm in any way I tried, using all my strength. The claim that non-full-contact martial arts can teach a weaker person to defeat a much stronger one is false. Even realistic training in modern martial arts is limited in how much it can improve a person’s chances of winning a fight.
Do unrealistic martial arts still help a little, e.g. enable a practitioner to win against a slightly stronger, or equal person? This is where the next story comes in.
After a year or two training karate twice or more per week, over 1.5 hours per session, I participated in a competition internal to the karate school (only members of that school took part). I was matched against a slightly smaller and weaker opponent who had only taken karate classes intermittently for a few months. In karate class, we had practised punches that ended an inch short of the opponent, to avoid hurting our training partners. I was a good student, as it turned out, because during the competition, all my punches ended an inch short of my opponent. I tried to overcome this in-trained behaviour, but could not. My opponent, on the other hand, had not been trained much and used the most common street-fighting overhand punches (called haymakers). These mostly landed on me and were moderately effective when they did. I lost badly.
The lesson from this is that bullshido actually reduces a person’s fighting effectiveness, unlike for example ballet or yoga, which train some aspects useful for fighting (strength, stamina, balance) and don’t ingrain ineffective moves. Unrealistic martial arts „help” a person lose against a smaller, weaker opponent, while falsely convincing the person of his or her fighting ability.

Urination technique 101

Many urinals are shaped so that if your centreline is aligned with the urinal’s centreline, then no matter where you aim, the spray splashes right back to your centre.

The solution is to stand slightly to one side, not directly in front of the urinal, but still aim straight ahead (perpendicular to the wall when viewed from above, not necessarily when viewed from the side). The urine starts flowing or splashing forward almost parallel to the wall of the urinal. This is the principle in a squash game – the more bounces off the wall, the slower the ball moves. If the energy for backward reflection is dissipated, then the ball falls down under gravity. The same applies to a urine stream – the more forward splashes off the wall of the urinal, the more energy dissipated and the more the urine flows downward.

Keeping journals honest about response times

Academic journals in economics commonly take 3-6 months after manuscript submission to send the first response (reject, accept or revise) to the author. The variance of this response time is large both within and across journals. Authors prefer to receive quick responses, even if these are rejections, because then the article can be submitted to the next journal sooner. The quicker an article gets published, the sooner the author can use it to get a raise, a grant or tenure. This creates an incentive for authors to preferentially submit to journals with short response times.
If more articles are submitted to a journal, then the journal has a larger pool of research to select from. If the selection is positively correlated with article quality, then a journal with a larger pool to select from publishes on average higher quality articles. Higher quality raises the prestige of a journal’s editors. So there is an incentive for a journal to claim to have short response times to attract authors. On the other hand, procrastination of the referees and the editors tends to lengthen the actual response times. Many journals publish statistics about their response times on their website, but currently nothing guarantees the journals’ honesty. There are well-known tricks (other than outright lying) to shorten the reported response time, for example considering an article submitted only when it is assigned to an editor, and counting the response time from that point. Assigning to an editor can take over two weeks in my experience.
To keep journals honest, authors who have submitted to a journal should be able to check whether their papers have been correctly included in the statistics. Some authors may be reluctant to have their name and paper title associated with a rejection from a journal. A rejection may be inferred from a paper being included in the submission statistics, but not being published after a few years. A way around this is to report the response time for each manuscript number. Each submission to a journal is already assigned a unique identifier (manuscript number), which does not contain any identifying details of the author. The submitter of a paper is informed of its manuscript number, so can check whether the response time publicly reported for that manuscript number is correct.
Currently, authors can make public claims about the response time they encountered (e.g. on, but these claims are hard to check. An author wanting to harm a journal may claim a very long response time. If the authors’ reported response times are mostly truthful, then these provide information about a journal’s actual response time. Symmetrically, if the journals’ reported response times are accurate, then an author’s truthfulness can be statistically tested, with the power of the test depending on the number of articles for which the author reports the response time.

Ideas for popular science experiments

There are many science fair experiment ideas online. The following may be repetitions.
A windmill connected to an electric generator, which is connected to a lightbulb (small dimmable is best). Blow on the windmill to turn the light on. A separate generator similar to the one attached to the windmill, which can be cranked by hand to turn on the lightbulb. An electric fan that can blow on the windmill, with power consumption at the fan and power production at the windmill measured and displayed. This explains efficiency losses in power generation.
Pressure of light. A piece of paper attached vertically on a platform that can rotate at low friction. On one side of the rotation axis, the paper is painted black, on the other, it is white. Reversed on the opposite side of the paper. Transparent dome over the setup to prevent air currents interfering. Shining a light on the paper makes it rotate, because the pressure on one side is greater. The rotating platform can be a polystyrene disk floating in water.
Pulleys and gear ratios. A bicycle with gears, rear wheel removed, but axle in place. Rope attached to axle, weight to rope. Rotating the pedals lifts the weight. Different gears require different number of rotations to lift the weight to a given height, but the more rotations needed, the less force needed for the rotation. Explain why low gears on a bike should be used when starting and on uphills, but high gears on downhills.
Moving pulleys: the distance the rope has to be pulled becomes longer, but the force required to pull becomes smaller to lift a given weight a given distance.
Friction in braking. Several bicycle wheels with brakes attached. Some rims are dry, some wet, some oiled. Feel the braking force required to stop each. To provide the force that the brakes must counter, a weight can be attached to each wheel. Rotate the weight away from the lowest point of the wheel and try to use the brakes to prevent the weight from sinking to the bottom again.
Friction in accelerating. Old bicycle. Rotate the pedals to accelerate the rear wheel to a given speed (measured with a bicycle speedometer) when the chain is dry, or oiled, or sand is poured on the chain. Feel the different difficulty depending on the condition of the chain. Several bikes is better, so the chains in different condition can be compared.
Friction and heat: a fire drill. Rotate a sharp stick in a hole in a piece of wood – the hole blackens and starts to smoke.
Ball bearings. Stack two blocks, put weight on the top one, try to rotate the bottom one. Now the same blocks with two ball bearings, one between the ground and the bottom bearing and the other between the two blocks. Much easier to rotate the bottom block. Explain how rolling friction is smaller than dragging friction. Friction proportional to pressure. Friction related to surface area.
Tire pressure and friction. Bicycle wheels (preferably with identical rims, hubs and spokes) with different width tires on them. Measure the friction of the tires by the force required to rotate them at a given speed on some surface, with the tire bearing weight. A stationary bicycle trainer can be used. Which tire width gives the lowest friction? Research shows that 22-23 mm tires have the lowest friction under the weight of an adult cyclist. Deflate the tire, measure the friction. Inflate, measure the friction. Which inflation pressure gives the lowest friction? Research shows that it is not the maximal pressure, unless the surface on which the wheel rotates is very smooth.
Mining and ores. Different rock samples of various ores. Panning for “gold”: try to find a shiny grain hidden in a quantity of mud or sand.
Solvents. Pebbles or small toys mixed in sugar paste or syrup, which is then dried into blocks. Use water to dissolve the sugar and discover what is inside the blocks.
Leidenfrost effect. Air hockey table with air being pumped under the hockey puck. Compare to droplets of liquid nitrogen rolling on a warm surface. Compare to pancakes riding on the steam bubbles under them on a hot pan.
Bridge of spaghetti. Dry spaghetti can be attached to each other with small balls of dough (flour mixed with water). Then bake the spaghetti-and-dough construction to harden the dough. Check how much weight the bridge can carry. Compare the breaking weight for a bridge (triangularly connected spaghetti that look like high-voltage power line towers) to the breaking weight for a bunch of horizontal spaghetti. The bridge can also be made by glueing matches or toothpicks.
Detergents. Lightly grease some cloth that normally absorbs water. Put water on top of that cloth – droplets form and nothing leaks through. Put water in a cloth bag and show that it does not leak. Compare to ungreased cloth that lets water through. Add detergent to the water on the greasy cloth. With the right coarseness of cloth, amount of grease and detergent, the water should start leaking through.
A burner under a thin paper box filled with water. The paper does not burn and does not become soggy, so the water does not leak through.
Absorption and radiation. Heat lamp shining on a black and white rock. Which becomes warm first? Now heat the rocks by contact, e.g. in warm water. Which rock cools down first? Infrared laser thermometer may help confirm temperatures. Or an ordinary thermometer inserted in a hole drilled in the rock.
Hot air rises. A nonflammable parachute rises above a candle. The parachute could be of thin tinfoil. It should have a light weight attached below the canopy to keep it upright. A tinfoil pinwheel can be held above the candle – it starts to rotate in the updraft of hot air.
3D printing by hand. Mud dripped from a hand onto sand or another surface that lets water through easily. The water soaks out of the droplets of mud that hit the surface, leaving a series of solid bumps of mud. Drop another droplet of mud on top of the bumps – water soaks out again, the bumps are now higher. Use this technique to build walls, castles etc.
Internal combustion engine. A rotating shaft with two pistons attached (can be made of wood or some other cheap, light, strong enough material). A balloon under each piston. If two people rapidly inflate and let deflate the balloons in the right sequence, then the shaft starts to rotate. Also a great party game – which couple gets their motor running fastest? The balloons can also be inflated by a hand pump, but fast and well-timed deflation is then a problem. One end of a long snakelike balloon can be under the piston, the other end squeezed and released by hand. The right sequence of squeeze and release can make the shaft rotate. One person can hold two long balloons, so can rotate the engine alone. A single piston is enough to rotate a wheel if attached to the rim – this is the arrangement in some old steam locomotives and foot-pedalled Singer sewing machines.
Steam power. Electric kettle boiling water, with a small windmill above the spout. The steam rising from the kettle rotates the windmill. Measure the power generated by the windmill and compare to the electricity required to heat the kettle.
Electric motor. Hand generator produces electricity, which is led by wires to an electric motor, which starts to rotate. Or the electricity is led to a coil with a permanent magnet inside. The magnet starts to move when the hand generator is cranked. With the right speed of cranking, an alternating current can make the magnet rotate.
Water mill. Pour the water at the top of a halfpipe. A waterwheel in the halfpipe starts to rotate in the flow. The rotation can drive a small generator which lights up a tiny lightbulb.
Archimedes’ screw. A helix in a pipe that is slightly tilted from horizontal can be rotated to pump water up the pipe.
Connected vessels. Two cups of water at different heights linked by a bent drinking straw. Suck the bottom of the straw to get water over the hump in the straw. Then water will start to flow from the top cup to the bottom, initially going uphill in the straw.
A bit of charcoal dropped in a vial of pure oxygen spontaneously catches fire. The oxygen can be generated at the bottom of the vial using hydrogen peroxide and a drop of blood. Steel wire in a pure oxygen environment rusts rapidly enough to be observed in a single experiment.
Bimetal thermostat. Two strips of different metals glued, soldered or riveted together at the ends. Heat the joined strip with a hair dryer – it bends to one side. The bent strip can touch a contact and switch something on or off, for example the hair dryer. A feedback loop can be constructed: if the joined metal strip gets cold enough, then it switches on the hair dryer, which heats the strip, which then switches off the hair dryer.
Tracks vs wheels. Tracked and wheeled model vehicles driving in a sandbox and on a smooth surface. The vehicles are the same weight, have the same motor and battery. Compare performance going up a sandhill vs racing on a smooth surface.
A spinning top to illustrate gyroscopes and self-stabilising.
Degrees of freedom of movement. Mechanical devices that illustrate the 3 translation and 3 rotation possibilities by allowing some of them, but not others. For example the serial gimbal, 3-axis gimbal, origami for thick materials (Science 24 Jul 2015: Vol. 349, Issue 6246, pp. 396-400, DOI: 10.1126/science.aab2870), bicycle front fork with shock absorbers, shopping trolley wheel or office chair wheel. For fun, rotate a person on an office chair a few dozen turns, then ask them to walk in a straight line.
Origami: Miura fold, hexaflexagon. Cutting and gluing mathematical objects, e.g. a Mobius strip, a Klein bottle. Math drawing with compass and ruler, e.g. a flower made from 7 interlinked circles of the same diameter. Sierpinsky carpet. Inscribing circles in squares and vice versa.
Boil amaranth in a transparent vessel. The grains dance on the bottom, then form columns, then a goo with large breaking and splattering bubbles.
Water volcano. A small bottle of warm coloured water is dropped in a large transparent vessel containing clear cold water. The coloured water will rise to the top and spread out, like a volcanic ash plume rises in the atmosphere and spreads in a layer.
Twinkling stars and heat mirages. A transparent rectangular vessel of water with points of light behind it. Look through the water: the points do not move. Now heat the water from the bottom: the points of light start to wobble, because warm water currents are rising up and bending the light passing through them.
Refractive index measurement. Water and cooking oil in transparent vessels. One person puts a straight stick at an angle into the liquid – the stick seems to bend at the immersion point. The person points out where they perceive the bottom end of the stick to be. Another person at the side of the vessel measures the angle between the perceived and actual sticks. The difference in angles is different for water than for oil.
Muscles and joints. Ask a person to relax their hand on a table, palm up. Press on the inside forearm and pull it toward the elbow – the 3 smallest fingers bend. Pull the palm towards the wrist – the fingers bend. An excavator’s scoop is moved by pistons like human limbs are moved by muscles – by pulling on the joint on one side or another.
Centrifuge to separate liquids, or solids from a liquid. The low-cost whirligig centrifuge (paperfuge) is described in Nature Biomedical Engineering 1, Article number: 0009 (2017) doi:10.1038/s41551-016-0009 and Animal blood or some mixture of liquids and solid flakes can be separated into component liquids and solids by spinning it.
Solar-powered distillation. The 1 square metre device made of polystyrene, paper and charcoal distils about 1 litre of water per hour, as described in Explain capillary action, evaporation and condensation, black material absorbing heat radiation faster.
Passive cooling. Glass beads 8 micrometres in diameter embedded in a polymethylpentene film backed with a thin silver film. The film conducts heat from the surface it sits on and radiates it away in in the other direction.
Oil- and water-repellent coating of glass. Cannot be prepared on the spot, but previously fabricated coatings can be demonstrated. Deposit candle soot on glass, then coat with a 25 nanometre silica film by putting the glass in a desiccator with open vessels of tetraethoxysilane and ammonia for 24 hours. Heat to 600C in air for 2 hours. Put in a desiccator with open beaker of semifluorinated silane for 3 hours. Xu Deng et al. “Candle Soot as a Template for a Transparent Robust Superamphiphobic Coating”

Electricity and water analogy. Pouring water down an inclined halfpipe to rotate a water wheel under the bottom of the halfpipe. The water wheel is connected to a winch that can lift a small weight. The height or angle of the halfpipe and the quantity of water can be changed, which lifts the weight at different speeds or can lift a larger weight. The height of the halfpipe is analogous to the potential difference (voltage) and the quantity of water per second to the current (amperage, electrons per second). The water wheel can also be connected to a generator that powers a small dimmable lightbulb. The brightness of the bulb changes with the speed of the water wheel.
Plastic beads in water poured down a halfpipe can demonstrate the measurement of flow per unit of time: the number of beads passing a given point per ten seconds for example.
Ship shape. Make model boat hulls out of some easily worked material, e.g. putty, model clay (hull must be thin to float), cut styrofoam, cardboard taped together and covered with cling film. The styrofoam needs extra weight, for example a piece of sheetmetal pushed through it in the middle to make a keel. The cardboard boat can carry some gravel as ballast. Then each boat gets a motor of equal power, e.g. a propeller powered by a wind-up spring from some toy. Weigh the boats and add ballast as needed to equalise weights. Race the boats to see which hull shape is fastest. To keep the racing boats straight, put them in parallel halfpipes of water, or create swimming pool lanes in a large tub by parallel strings drawn taut on the surface of the water.

Priming the pump. An interesting mechanical device is a piston pump for getting water out of the ground. A piston moves up and down in a vertical pipe open at the top. There is a hole in the side of the pipe from which water can come out. The piston passes above and below the hole as it moves. A dry piston doesn’t pump water up, but pouring some water on the piston (priming the pump) creates a temporary airtight seal around the piston, so pulling the piston up creates a slight vacuum under it, which draws the water up. The water flows out of the hole in the side of the pipe.
Spinning methods and yarn properties. Threads or wires can be spun into yarn by cone spinning, Fermat or dual-Archimedean spinning. With enough twist, the yarn starts to coil like a telephone cord. Folding twisted yarn in two makes the two halves twist around each other – this is how rope is made. Twisted but uncoiled yarn can be coiled in the opposite direction to the twist. Removing the core around which the coiling took place lets the twist and coil cancel – parallel threads result.

Camouflaged encryption

Many governments (US, Australia, all dictatorships) want to make end-to-end encryption illegal and prevent IT firms from providing it. The open-source community can create their own encryption software, but the creators and users of this could be punished as well. The reasoning of the governments for banning encryption is that criminals and terrorists use it. However, the same reasoning applies to knives, guns and cars, which are used much more directly to harm people and yet are strangely excluded from the ban. This contradiction makes me doubt the motives of these governments.
The obvious solution to a ban on some software is to camouflage it and its products. The code for the encryption software could be hidden in a seemingly nonexistent part of computer memory or blended in one log file among many, perhaps encrypted as well.
The encrypted messages passing through the internet should not look like encrypted messages, but would be embedded in innocuous-looking files. A simple way is to change the colour of some pixels in a self-made photo or video file, with the locations of the relevant pixels being known to the sender and receiver, but secret from others. The colours of the pixels can encode the data. Someone intercepting the picture or video would have to spend significant resources analysing it to find whether some pixels are of an unusual colour, especially if the starting image is riotously colourful and confusing. Publicly available images are not useful, because comparing the message-image with the original reveals the changed pixels.
A more sophisticated version of this idea has already been done by A similar idea is to hide one’s browsing history in random websurfing (, but this only hides the relative frequencies of websites visited, not the fact of visiting a site on a government watchlist that most people don’t visit.

Silly balconies

Everywhere in Australia, I have seen buildings with balconies that overlook busy roads. The view from the balcony often only includes other buildings. These balconies seem useless, because not many people want to sit in the street noise and car exhaust. I have rarely seen anyone on these balconies, and then only moving around for a practical purpose, not enjoying the air and view. Mostly the balconies are used for storing unwanted furniture and sports equipment, or growing potted plants. This makes sense, because even drying laundry over a busy road is problematic – everything gets covered in fine black soot. What does not make sense is adding these balconies to the buildings in the first place. A more practical use of the space would be to close the open parts of the balcony and thus add an extra room to the apartment. It is used as a storage room anyway.

In some cases, the building might have been constructed before the street became too busy or the views blocked by other buildings, but most of the buildings with balconies are new, so this explanation does not apply.

The reason the developers add balconies to their buildings is probably to market the apartments to impractical people. An included balcony makes the apartment sound more luxurious, and usually the view and relaxation opportunities of the balcony are touted in the advertisement. But people inspect the apartment before buying, so they should see the uselessness of the balcony for anything but storage. Inspections are usually scheduled on Saturdays when there is less traffic, and the inspecting buyers don’t sit on the balcony for long enough to become annoyed by the noise and the car exhaust.

There may be rules against the owner of an apartment closing up the balcony to create a room, because this makes the building facade uneven. Coordination problems between apartment owners may prevent them from closing up all the balconies of the building simultaneously.