Knowledge library
1037 science cards across our topics. Choose a topic to browse every card, with its full explanation, method, limitations and sources.
Brain and body · 111 cards
Why does a chili pepper feel hot when it never warms your mouth?
Eat a chili and the temperature in your mouth barely moves, because there is no source of heat anywhere in it. The sensation is still close to a burn. So what exactly is being switched on?
Source: Caterina et al., Nature
What does the brain do at night that it cannot get done by day?
Every tissue in the body has a lymphatic system to carry waste away, and the brain has none, although it burns about a fifth of the body’s energy. To see how it clears anyway, researchers injected a glowing tracer into the spinal fluid of mice and filmed the living brain awake and asleep. What changed between the two?
Source: Xie et al., Science
Who decides which of today’s memories you still have tomorrow?
Through the day the hippocampus takes in new experience fast, and it is not built to hold any of it for long. Something has to move what is worth keeping somewhere else. When does that happen?
Source: Rasch and Born, Physiological Reviews
Space · 74 cards
What happened to an astronaut’s genes after a year in space, next to his identical twin on the ground?
Scott Kelly spent close to a year on the space station while his identical twin Mark, himself a retired astronaut, stayed on Earth. That is about as good a control as human biology ever offers. What did the comparison show?
Source: Garrett-Bakelman et al., Science
Why is every look at the sky a look into the past?
Light has a finite speed, which sounds like a technical footnote. But if it left a star a thousand light-years away, it reaches your eye a thousand years later. What does that make of what you are seeing right now?
Source: NASA Science
How is the distance to the Moon known to the millimeter?
The tides are raised by the Moon’s gravity, and the relationship runs both ways: the moving water lags slightly behind the Moon and tugs it forward along its orbit. Which means the Moon is leaving. How do you measure something like that?
Source: Dickey et al., Science
Nature · 149 cards
Where is most of an octopus’s brain?
In a human, nearly all the processing happens in one place, which looks like the sensible way to build an animal. An octopus is arranged differently enough that a single arm can feel its way along and settle how to grip. So where is the brain?
Source: Hochner, Current Biology
Do trees really help each other underground?
The roots of most trees are wrapped in fungi that supply minerals and take sugars in return, and one fungus can be attached to several trees at once. Out of that grew a story about a forest where trees feed and warn one another. How much of it holds?
Source: Karst, Jones and Hoeksema, Nature Ecology and Evolution
Which animal came back alive from open space?
When its surroundings dry out, a water bear does not die. It shrinks, pushes almost all the water out of itself, and enters a state where its metabolism nearly stops. In that state it withstands extreme cold, drought, pressure and radiation. So they flew one to orbit and opened the hatch.
Source: Jönsson et al., Current Biology
Technology · 80 cards
Did a brain scan prove that writing by hand teaches better than typing?
The headline went everywhere: a brain scan had settled it, and handwriting beat typing. Underneath sat a real result, wider connectivity across the brain in EEG while people wrote by hand. Then a published critique pointed at the step from that result to the claim. What was missing?
Source: Pinet and Longcamp, Frontiers in Psychology
What would happen to the navigation on your phone without relativity?
A navigation receiver works out where it is by trilateration: from minute differences in the arrival times of signals from several satellites. So the accuracy of their clocks is everything. Relativity says two opposite things about such a clock: its high speed should slow it down, and the weaker gravity up there should speed it up. Which one wins, and what does it cost?
Source: Ashby, Living Reviews in Relativity
What does the router in your house know about who is in the room?
A Wi-Fi router sends out radio waves that bounce off walls, furniture and people, which gives the receiver several copies of the same signal arriving by different paths. The receiver is already measuring the strength and the shift of each one at every frequency. When somebody moves in the room, that pattern changes. What can be read out of it?
Source: Adib & Katabi, ACM SIGCOMM
Psychology · 130 cards
Why is it so hard to say what color a word is printed in?
Take the word "blue", print it in red ink, and say the ink color out loud. Nothing about that is difficult, and most people still slow down, stumble and get it wrong. Why does an easy task fight back?
Source: Journal of Experimental Psychology, Stroop 1935
Why did the people who paid full price show up to more plays?
Hal Arkes and Catherine Blumer stopped people buying season tickets to a theater series and sorted them at random into three groups: full price, small discount, large discount. The discount came out of a draw and reflected nothing about the plays. Who came to more of them?
Source: Arkes and Blumer, Organizational Behavior and Human Decision Processes
What happened when the Dunning-Kruger effect was run on random data?
The Dunning-Kruger effect, as the 1999 paper put it, is that poor performers rate themselves too highly, because the knowledge they lack is the same knowledge that would let them see it. The chart behind it is persuasive. Then someone fed the same method data with nothing in it at all. What came out?
Source: Gignac and Zajenkowski, Intelligence
Environment · 94 cards
If trees cancel almost half the urban heat effect, why does the thermometer barely move?
Two numbers about the same city trees sound like they are arguing. One says the trees cancel close to half of the urban heat effect. The other says they pull the thermometer down by a fraction of a degree. Both are correct, so how do they fit together?
Source: Nature Communications
Does the Amazon still absorb more carbon than it releases?
A forest takes in carbon as its trees grow and gives it back when they burn or rot, so the balance can flip, and not in the same way everywhere. To find where the Amazon stands now, researchers measured the air above different parts of it and compared them. What came out of each region?
Source: Gatti et al., Nature
What comes out of the frozen ground of the north when it thaws?
Permafrost is ground that stays frozen for years on end, and it covers vast areas of Siberia, Alaska and Canada. Locked inside it are the remains of plants and animals that never got the chance to rot. What happens when it thaws?
Source: Schuur et al., Nature
History of science · 86 cards
How did meteorites make it possible to measure the age of the Earth?
Measuring the age of the Earth directly is hard, because rock at the surface melts, erodes and is replaced without pause. The oldest stone you can find here is younger than the planet. So where do you get material that never changed?
Source: Patterson, Geochimica et Cosmochimica Acta
The bowerbird lays out stones in front of its bower. Why do they get smaller?
A male bowerbird builds an avenue of sticks with a court in front of it covered in stones, bones and shells, and arranges them by size: small ones near, large ones far. When researchers shuffled them, he restored the order within days. Why does it matter to him?
Source: Kelley & Endler, PNAS
How did knots in a cord hold numbers with no written digits?
A quipu is built from a main cord with colored strings hanging off it, tied with knots. There are no digits there, no ink and no page. And still an exact number can be recorded in it. How does that work?
Source: Ascher & Ascher, Nature
Mathematics · 56 cards
How many people does it take before two of them share a birthday?
There are 365 days in a year, so it feels like a room would need hundreds of people before two birthdays collide. But the thing being counted is not people, it is pairs, and every pair is its own chance to match. So how big does the room have to be?
Source: Britannica
How long is the coast of Britain?
Measuring a coastline means first choosing a ruler, which sounds like a technical footnote. A ruler 100 kilometers long gives Britain roughly 2,800 kilometers of coast. Halve the ruler and the answer climbs to roughly 3,400. So how long is the coast?
Source: Britannica
Is there a way to stack oranges more densely than the greengrocer does?
Greengrocers stack oranges in layers where each fruit sits in the hollow between the fruits below it, without thinking about it twice. Kepler’s conjecture, from 1611, said no other arrangement of identical spheres fills more space. How long did proving that take?
Source: Hales, Annals of Mathematics
Archaeology · 71 cards
What was found in the village where the pyramid builders lived?
The familiar story of slaves driven to build the pyramids comes from sources written far later, not from the period of construction. At Giza, excavators found a large settlement right beside the pyramids, and the builders lived in it. What was in it?
Source: Murray, Archaeology International
Does every one of the 8,000 terracotta soldiers really have its own face?
The First Emperor’s terracotta army runs to more than eight thousand clay soldiers, and the line everyone repeats is that no two faces are alike. Sculpting eight thousand different faces by hand is close to impossible. So how was it done?
Source: Quinn, Zhang and Xia, Antiquity
What is hiding under the jungle of Guatemala?
Survey aircraft fire laser pulses at the ground and measure how long each takes to return. Some of the pulses find gaps between the treetops and reach the soil, so filtering out the vegetation leaves a model of the ground itself. What came up under the jungle?
Source: Canuto et al., Science
Belief · 42 cards
Does meditation actually change the connections inside the brain?
Saying that meditation changes the brain sounds like too large a claim, and it is hard to test without measuring something directly. In one small trial, adults under stress were assigned at random either to three days of mindfulness training or to three days of relaxation. What separated the two?
Source: Creswell et al., Biological Psychiatry
Why can repeated prayer change the rate of your breathing?
Repeating words in rhythm sounds like a purely mental act, with no bodily side to it. To check whether that is right, participants recited a Latin prayer and a yogic mantra while instruments tracked their breathing, blood pressure, heart rate and cerebral blood flow. What happened to the body?
Source: Bernardi et al., BMJ
Can a painful ritual raise generosity toward a community?
At the Thaipusam festival a mild ritual and a severe one, which involves piercing the body and carrying weights, run side by side. At the end participants were given money and decided for themselves how much to donate to the temple, and onlookers were asked too. What separated the groups?
Source: Xygalatas et al., Psychological Science
Sport · 76 cards
Does an ice bath after training actually speed up recovery?
An ice bath after a hard session feels like treatment, which is a good reason to be suspicious of it. A review pooled dozens of trials against plain rest and kept explosive power, maximal strength, soreness and how recovered people felt as four separate questions. Which of them moves?
Source: Moore et al., Sports Medicine
A player has scored three in a row. Is it really worth passing to him again?
For years it was taught that the hot hand is an illusion, and that a previous score does not raise the chance of the next one. Except that the way shots after a streak get selected hides a mathematical bias, so researchers corrected it and reanalyzed three-point contests. What came out of the correction?
Source: Miller & Sanjurjo, European Economic Review
Why does a curling stone turning right also drift right?
You would expect a stone spinning right to drift left, like a spinning ball in the air. In curling the opposite happens: the stone drifts the way it is turning. The stone rests on nothing but a narrow ring. What is going on down there?
Source: Honkanen et al., Scientific Reports
Art · 68 cards
Why does almost every language name red before it names blue?
Almost every language has a word for red before it has one for blue, and that holds even between languages with no contact at all. The obvious explanation is the eye, except the eye is the same everywhere. One study measured how speakers of three very different languages describe the same color chips. What came out?
Source: Gibson et al., PNAS
Why is a ballet dancer not dizzy after twenty turns?
Deep in the inner ear sit fluid-filled canals that measure rotation. Stop turning and the fluid keeps moving, so the brain reads motion that is no longer happening, and that reading is dizziness. A dancer comes out of twenty turns steady. Why does the same signal not topple her?
Source: Nigmatullina et al., Cerebral Cortex
Can an audience in a hall really hear a Stradivarius?
A Stradivarius, and its Guarneri rivals, are credited with a rare power to fill a hall, and their prices are set on it. To test that, soloists played old and new instruments from behind a screen while hundreds of listeners rated what they heard, not knowing which was which. What did they choose?
Source: Fritz et al., PNAS