Thoughts

Wombats leave droppings shaped like cubes, and for a long time nobody knew how, because the exit is round like everyone else’s. The answer, published by Patricia Yang and her colleagues at Georgia Tech in 2021, is that the corners are made earlier, in the gut. The wall of a wombat’s intestine is not the same all the way round: some bands are thicker and several times stiffer than others, and as it squeezes, the soft stretches give and the stiff ones hold, pressing flat faces with edges between them. The cube only sets near the very end, in roughly the last twelfth of the journey, dry enough by then to keep its shape on the way out. Factories make cubes by moulding or cutting. The wombat found a third way, which is to squeeze unevenly on purpose. It is the opposite of the old saying: here is a square peg, and it passes through a round hole without any trouble, because it was square before it got there.

Stale bread isn’t dry bread. You can seal a loaf in a bag so tightly that it keeps almost every drop of its water, and it will still go hard. What changes is the starch. In the oven its tidy crystals swell with water and come apart; as the loaf cools they slowly fold back into order, holding the water tightly in their structure, where it no longer makes the crumb soft. The process has a temperature range, and a fridge sits right in the middle of it, which is why a loaf kept in the cold goes stale several times faster than one on the counter, while a freezer, below the range, nearly stops it. The good part is that the change runs both ways. Put a stale loaf back in a hot oven for a few minutes and the crystals melt again, and for an hour or so it is close to new. Very few kinds of going-off can be undone. Staleness is one of them, because nothing was lost; it was only put away too neatly.

In 1994 a park ranger named David Noble abseiled into a gorge in the Blue Mountains, about 150 kilometres from Sydney, and found a tree nobody had seen alive: the Wollemi pine, whose pollen matches fossils from the middle of the age of dinosaurs. Fewer than a hundred adults grow wild, and where they grow is still kept secret, because one careless visitor could carry a disease in on their boots. So the people protecting it did two opposite things at once. They hid the wild trees, and they made the species as common as they could: they grew young trees from cuttings, auctioned the first ones at Sotheby’s in 2005, and now you can buy one at a garden centre. When the Black Summer fires reached the gorge in 2019, firefighters were lowered from helicopters to lay irrigation lines, sworn never to say where they had been. The trees survived. I like the arrangement: the most hidden place in a national park and an ordinary pot on somebody’s patio, keeping the same tree alive from both ends.

A deck of 52 cards can be put in order in about 8 followed by 67 zeros ways, so many that a properly shuffled deck is almost certainly in an order no deck has ever been in before. The catch is the word properly. In 1992 Dave Bayer and Persi Diaconis, a mathematician who had left home at fourteen to travel with a card magician, worked out how many ordinary riffle shuffles it takes to get there. Their answer was seven. The surprising part is how it arrives: after four or five shuffles the deck still keeps a lot of its old order, then around six and seven it gives way almost at once. Most of us stop at three, which feels like plenty. Thoroughness turns out not to be a dial you can turn halfway; for a while nothing much happens, and then suddenly it’s done.

Pour a stout and watch the side of the glass while it settles: the bubbles drift down. Bubbles can’t sink, and these don’t. In 2012 three mathematicians at the University of Limerick, Eugene Benilov, Cathal Cummins and William Lee, worked out what is going on. A pint glass is wider at the top, so its wall leans outward. Every bubble rises straight up, which means the ones near the wall move away from it, leaving a thin layer of beer by the glass with no bubbles in it. Beer without bubbles is heavier than beer with them, so that layer slides down the wall, and the bubbles beside it are so small that they get carried along. Up the middle, the rest of the pint rises to make room. Put the same stout in a glass that narrows towards the top and the whole thing runs the other way: the bubbles by the wall go up, like any sensible bubble. What you were watching was never the bubbles. It was the beer, and the shape of the glass told it which way to go.

Mapmakers used to plant small lies to catch thieves. Some time before the Second World War, Otto Lindberg and Ernest Alpers of the General Drafting Company put a town called Agloe, an anagram of their initials, at an empty crossroads in the Catskills, on a road map they made for Esso. If Agloe ever turned up on someone else’s map, they would know it had been copied. It did turn up, on a Rand McNally map, and the story goes that when the accusation came, Rand McNally answered that Agloe was real: there was a general store at that crossroads now, and it was called the Agloe General Store. Its owners had taken the name from the map.

I like that the trap was sprung by the one thing it assumed could never happen: someone looked at a town that wasn’t there and believed in it enough to open a shop. The store is long gone. The name stayed on maps for decades anyway, a place that was invented, then real, then invented again.

OK began as a joke, and not a very good one. In 1839 Boston newspapers had a fad for abbreviating phrases that were themselves misspelled on purpose, the way a country bumpkin might write them, and on 23 March the Boston Morning Post printed “o.k.”, for “oll korrect”. Nearly all its cousins have vanished. This one was rescued by an election: in 1840 supporters of Martin Van Buren, who came from Kinderhook, New York, and was called Old Kinderhook, formed O.K. Clubs, and the letters picked up a second meaning just when they needed one. Then everyone forgot the joke and kept the word. The linguist Allen Walker Read had to spend the 1960s going through old newspapers to find out where it came from. It is often called the most widely understood word on earth, and it stands for two spelling mistakes.

There has never been any lead in a pencil. The black stuff found at Borrowdale, in the Lake District, in the sixteenth century looked enough like lead ore that people called it black lead, and it got a name of its own only in 1789, when the geologist Abraham Werner took one from the Greek for “to write”. Borrowdale’s was pure enough to saw into sticks, and valuable enough that in 1752 Parliament made stealing it a felony; later the miners were searched at the end of every shift.

Then Britain and France went to war, and France could not buy any. In 1794 the war minister, Lazare Carnot, asked Nicolas-Jacques Conté for a pencil that needed no English graphite. Conté ground up poorer graphite, mixed it with clay and baked it, and found that the more clay he added, the harder the point and the paler the line.

So the H and the B on the side of a pencil, the whole run from a draftsman’s 4H to an artist’s soft 8B, come out of a blockade. The best graphite in the world made one kind of pencil. Having none of it made all the others.

Until 1883 every American town kept its own noon, the moment the sun stood highest over that particular street, so a train crossing the country passed through dozens of slightly different times without anyone touching a clock. On Sunday, 18 November 1883, the railroads fixed that by themselves. A telegraph signal went out when it was noon on the 90th meridian, and station clocks were reset to the new zones. In New York, which sits a little east of its zone’s line, noon came by the sun, and then about four minutes later it came again by the railroad. People called it the day of two noons. Nobody had passed a law. Congress made the zones official only in 1918, so for thirty-five years the country told time by a timetable a few companies had agreed on to keep their trains from running into each other, and hardly anyone noticed there was no law behind it.

Robert FitzRoy, who had captained the Beagle while Darwin collected finches, became Britain’s first official meteorologist, and in 1861 he started telling the public what the weather would do. He did not want to call it a prediction. Prediction and foretelling belonged to almanacs and fortune-tellers, and he was working from barometer readings sent in by telegraph. So he chose a plainer word, forecast, the way you might cast a line ahead of you and see where it lands. The first one ran in The Times on 1 August 1861.

It was mocked whenever it missed, which was often. FitzRoy died in 1865. A committee of scientists who had never trusted his methods reviewed the office and recommended it stop forecasting and go back to collecting numbers, and in May 1866 the public forecasts ended.

Then the complaints changed direction. Sailors and fishermen wanted the storm warnings back, and got them in 1867. The daily forecasts took until 1879. It seems to be how people treat anyone who tells them what is coming: they grumble at every wrong guess, and then they find out what it is like to step outside with nothing to go on.

Birders in Canada learn the white-throated sparrow’s song as “oh sweet Canada, Canada, Canada”: two long whistles, then a run of three-note triplets. That was the song in the 1960s, across the whole country. In 1999 the ornithologist Ken Otter moved to Prince George, in northern British Columbia, and noticed the local sparrows were cutting the last word short, into two notes instead of three. Over the next twenty years the clipped ending spread east more than 3,000 kilometres, and by 2019 it had all but wiped out the old one from British Columbia to central Ontario.

The part I keep turning over is how it travelled. Otter’s team put tiny trackers on fifty sparrows from Prince George and found they spend the winter in the same places in the southern United States as sparrows from farther east. So a young bird from Alberta can hear the new ending from a stranger a long way from home, and carry it back north in the spring.

They have been singing the country’s name for as long as anyone has listened, and the newest way to say it, they picked up abroad.

Work on the bell tower in Pisa began in 1173, on ground that turned out to be soft clay. Around 1178, with three storeys up, it had already started to lean, and then the city went to war and the builders went away. They came back almost a hundred years later. By then the clay had been pressed down under the weight and had firmed up enough to carry more. Engineers who have studied it think that if the work had gone on without a break, the tower would have fallen over before it was finished.

The second crew couldn’t straighten it, so they made the storeys taller on one side, trying to bring the top back over the middle. That’s why the tower isn’t straight but curves slightly, like a banana. Then there was another war and another long stop, and the bells went up in 1372, about two hundred years after the start.

What I like is that the interruptions did the real work. Nobody planned to wait a century. The wars that stopped the building were what saved it. When it was finally made safe, in 2001, the engineers did it slowly too: they dug soil out from under the high side, a little at a time, over about ten years.

In a church in Halberstadt, Germany, an organ is playing a John Cage piece called Organ²/ASLSP, for “as slow as possible”. The performance began on 5 September 2001 with a rest, and the rest lasted seventeen months; the first sound came in February 2003. The chord changed for the seventeenth time on 5 August 2026, when more than 150 people came to hear one more pipe join in. The next change is on 5 October 2027. The last note is due on 4 September 2640.

You can already buy a ticket for the ending. It costs 2,640 euros and it is inheritable, which makes it less a ticket than a letter: something handed down through a family for six centuries with a note saying where to sit. Nobody who was there at the beginning will hear the end, and nobody at the end will have heard the beginning, though in fairness the beginning was silence. The piece only exists whole the way a cathedral does, or a family, as something carried by people who each get a little of it. The crowd that turned up in August was there for the same reason the one in 2640 will be. It was their turn.

Why is the sky dark at night? It sounds like a child’s question, and it is, but it bothered Kepler in 1610 and the German astronomer Wilhelm Olbers in the 1820s. If the stars go on forever, every line of sight should end on one, and the whole sky should blaze like the surface of the sun.

The first answer that holds up came not from an astronomer but from Edgar Allan Poe, in Eureka, a long prose poem about the universe that he first gave as a lecture in New York in February 1848. The dark gaps between the stars, he suggested, are where the light hasn’t arrived yet: the background is so far off that no ray from it has had time to reach us. Lord Kelvin worked the same idea through properly in 1901, and cosmologists now agree it is the heart of the answer. The universe has an age, so when we look out we are looking back, and the past runs out.

Poe thought Eureka was the most important thing he would ever write. Most readers took it for the odd late work of a man best known for a raven. He was right about the sky anyway. Darkness isn’t always emptiness. Sometimes it’s light that is still on its way.

Lewis Fry Richardson, a Quaker who had driven ambulances in the First World War, spent the years after it trying to find out mathematically why countries go to war. One idea was that neighbours with longer shared borders fight more often, so he went to look up the lengths. Portugal said its border with Spain was 987 kilometres. Spain said 1,214. Neither had made a mistake. They had measured with different rulers, and on a wiggly line a shorter ruler fits into more of the wiggles, so the total keeps growing the closer you look. A border like that has no true length. Its length depends on how carefully you measure it. Richardson died in 1953 without finding his law of war. His notes on the rulers came out in 1961, and in 1967 Benoit Mandelbrot built on them in a paper called “How Long Is the Coast of Britain?”, which was a starting point for fractals. So the man who went looking for what divides countries found that the line between them can’t be measured, and I think he would have liked that.

For most of the twentieth century chemists thought onion tears were an accident. Cutting an onion breaks its cells, an enzyme called alliinase meets the sulfur compounds it had been kept apart from, and the stinging gas was supposed to fall out of that reaction on its own, the way sawdust falls out of sawing. Then, around 2002, Shinsuke Imai and his colleagues at House Foods in Japan tried to make the gas in a dish, using onion compounds and alliinase taken from garlic. Nothing happened. Garlic has alliinase too, and garlic has never made anybody cry. The missing piece turned out to be a second enzyme, which onions have and garlic doesn’t, whose only known job is to turn the sawdust into the sting. So it was never an accident. The onion spends a whole enzyme on it. The experiment that found this was the one that failed, and in 2013 it won an Ig Nobel Prize for showing that crying over onions is more complicated than anyone had thought.

Around 1800 the French botanist René Desfontaines took a potted mimosa for a ride through Paris in a horse-drawn carriage. Mimosa pudica folds its leaves shut at the slightest touch, and when the carriage started jolting it closed up as expected. Then, as the ride went on, it opened again and stayed open, bumping along like a passenger who has decided the road is just the road. He mentioned it briefly in a book about French trees and shrubs, and that was that for about two hundred years.

In 2014 the ecologist Monica Gagliano did the experiment properly. She dropped potted mimosas about fifteen centimetres, sixty times in a row, and by the end most had stopped folding. They weren’t tired: shake one sideways instead and it shut at once. They had sorted the fall into the category of things that happen and don’t matter. Tested again twenty-eight days later, they still knew.

A plant has no neurons, so wherever it keeps that, it isn’t where we keep ours. What I like is the shape of the lesson it learned, which is the hard one: not “this is dangerous” but “this, specifically, is not”. Most of growing up is learning which of the jolts to stop flinching at.

One of two shapes is called kiki and the other bouba: a spiky star, or a soft blob like a cloud. You already know which is which. In 1929 the psychologist Wolfgang Köhler asked readers the same thing with two made-up words, takete and maluma, and in 2001 V. S. Ramachandran and Edward Hubbard swapped in kiki and bouba and found that about 95 per cent of people gave the spikes to kiki. A 2022 study of 917 people speaking 25 languages found the same pull almost everywhere.

The tidy explanation was that this is something a mind built for language does: the lips round for bouba, the tongue snaps for kiki, and we hear the shape of our own mouths. Then in February 2026 a team in Padua played the two sounds to chicks less than a day old, chicks that had never heard anyone speak, and the chicks spent more time by the spiky shape when they heard kiki and by the round one when they heard bouba.

So the match may be older than words, older than us, as old as whatever we share with a chicken. I like that. Part of what a word means it brings with it, before anybody has agreed what it means.

Monopoly began as a warning. In 1904 a Washington stenographer named Lizzie Magie patented The Landlord’s Game: a square track of properties, rent paid to whoever owned the square you landed on, and a corner that sent you to jail. She meant it to show what happens when a few people own the ground everyone else has to cross. She gave it two sets of rules. Under one you won by bankrupting everybody else. Under the other, later called Prosperity, every player gained whenever anyone bought new land, and everybody won together once the player who started with the least money had doubled it. In 1935 Parker Brothers bought her patent for $500 and put its weight behind the version Charles Darrow had been selling, the one with only the first set of rules, which he said he’d invented in his basement. So the game about who ends up with the money ended up with somebody else. The rules she left behind are the more interesting game now, I think: not because winning together is sweeter, but because it is so much harder to arrange.

In 1998 a Forest Service scientist named Catherine Parks went looking for whatever was killing the fir trees east of Prairie City, Oregon. She took root samples from 112 dying trees, grew the fungus from each one in a dish, and then set the cultures side by side in pairs. Two cultures of honey fungus from different individuals refuse each other: where they meet, a line forms between them. Two from the same individual simply grow together. Sixty-one of hers grew together, from trees a long way apart. The thing killing the forest was one fungus, Armillaria ostoyae, spread through about 2,385 acres of soil, nearly four square miles, and somewhere between 2,400 and 8,650 years old. It may be the largest living thing anyone has found.

Nearly all of it is underground, in dark threads that run through the soil from root to root. What shows above ground, each autumn, is a few clumps of ordinary brown mushrooms at the foot of a tree, the kind you would walk past. I like that the way to measure something too big to see was to take small pieces of it, two at a time, and ask whether they recognised each other.

Nobody in olden times ever said “ye olde”. In a sign like Ye Olde Tea Shoppe the Y is standing in for a lost letter, thorn, þ, which spelled the sound at the start of “the”. By the fourteenth century scribes had worn it down until it lost its tall stem, and when printing reached England in the 1470s the type came from the Low Countries, cut for Latin, French and Dutch, with no thorn in the case. Printers used the letter that looked nearest, which was y. So “ye” was read “the” for well over a century; the first King James Bible, in 1611, still prints it in places, along with “yt” for “that”.

What I like is how completely the stand-in won. Thorn faded out of English, and once nobody remembered it, readers took the y at face value and gave the word a new sound. The best-known antique word in English is a typesetting workaround, mispronounced. Thorn itself is still alive in Icelandic, where it begins the way you say thank you: þakka þér.

When J. R. R. Tolkien was about seven he wrote a story about a dragon. Years later he remembered only one thing about it, and he put it in a letter to W. H. Auden in 1955: his mother said nothing about the dragon, but pointed out that you couldn’t write “a green great dragon”. It had to be “a great green dragon”. He wondered why, he wrote, and still did. He was a professor of English by then.

Grammar books now give a rough answer, an order that runs opinion, size, age, shape, colour, origin, material, purpose. That is why you can own a lovely small old round blue Dutch china teapot, and why moving any one of those words makes it sound like a translation. Almost nobody who speaks English was ever taught the order. It sits underneath, like a floor nobody remembers laying, and you only find it when you put your foot through it, the way a seven-year-old does.

What I like is that naming the rule doesn’t explain it. Tolkien knew the history of English as well as anyone alive, and the honest thing he could say about that one sentence was that he still wondered.

Charles Babbage was twice asked by members of Parliament, “Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?” In his memoir of 1864 he said he could not rightly understand the confusion of ideas that would provoke such a question. For his engine he was right. But the question didn’t go away. In 1947 Richard Hamming at Bell Labs was only allowed the relay computer at weekends, so he would set a job running on Friday evening and come back on Monday to find that one error had made the machine abandon the whole thing. He reasoned that if a machine could tell an error had happened, it ought to be able to tell where, and put it right. In 1950 he published the codes that carry his name, where a few extra check bits let a machine find a single wrong figure and fix it. That answers the question with a qualified yes: one wrong figure, in a known place, if you planned for it in advance. Most good listening works like that. When someone knocks with a question that has a wrong figure in it, the right answer is often still there to be found, so long as I care more about what they meant than about what they typed.

In February 2006 the biologist John Marzluff and his team caught seven wild crows on the University of Washington campus to band them, and while they did it they wore a rubber caveman mask. Afterwards, anyone who walked across campus in that mask got scolded and mobbed from the trees, while someone in a different mask on the same path was left alone. That much you might expect. The surprise came over the following years: the mobbing spread. Crows that had never been caught joined in, and so did fledglings that hadn’t hatched when the net came down, because they had watched their parents yell at that face and learned it. The campus crows kept it up for well over a decade. A grudge, in a crow, turns out to be less a memory than a piece of local knowledge, handed down like the name of a bad road.

Each of your eyes has a hole in it. Where the optic nerve leaves the back of the eye there are no light-sensitive cells at all, and the gap sits about fifteen degrees out from the centre of your gaze, toward your temple, wide enough to hold more than ten full moons side by side. Nobody knew it was there until the 1660s, when the French physicist Edme Mariotte worked out from dissections where a blind patch ought to be and then went looking for it, which is strange when you think how long people had been looking at things. It hides for two reasons: the other eye covers it, and the brain fills what is left with whatever surrounds it, so a blank wall stays blank and a patterned carpet stays patterned. Close your left eye and look at one word on this screen with your right, and a patch somewhere off to the right of it is being made up, convincingly, as you read.

A star rises about four minutes earlier every night. The Earth turns once against the distant stars in 23 hours and 56 minutes; the clock’s other four minutes go on catching up with the Sun, because in a day we have moved about a degree along our orbit and have to turn a little farther to bring it back to the same place in the sky. Four minutes a night doesn’t sound like much, but it comes to two hours a month, and over a year to one whole extra turn: the Earth spins about 366 times against the stars for every 365 sunrises. That one uncounted day is why the winter sky isn’t the summer one. Orion hasn’t gone anywhere in July. He’s up there in the daylight, rising and setting with the Sun, while the four minutes bring him round again.

A small fair-weather cloud, the cotton kind about a kilometre across, holds something like five hundred tonnes of water. That is the meteorologist Peggy LeMone’s back-of-the-envelope figure: about half a gram of water in every cubic metre, times a billion cubic metres, which comes to roughly the weight of a hundred elephants hanging over a picnic. It stays up because it isn’t really floating. Each droplet is about a fiftieth of a millimetre across and falls through the air at around a centimetre a second, but the warm air rising underneath is climbing faster than that, so the whole cloud is like someone walking down an escalator that is going up. In a warm cloud, rain begins when droplets bump into each other and merge until they are heavy enough to outwalk the escalator.

The first gutta-percha golf balls, from the 1840s, were smooth, and players noticed something odd about them: an old ball, nicked and scuffed from a season of use, flew farther than a new one. Ball makers started hammering patterns into new balls on purpose, and dimples grew out of that. Physics took longer to explain why. Air flowing over a smooth ball peels away early and leaves a wide wake of low pressure that pulls the ball back. Dimples churn the thin layer of air nearest the surface into turbulence, and turbulent air clings to the curve longer before it lets go, so the wake is narrower and the drag falls by about half. The damage was the design all along. Someone just had to notice the ball flying better for being worn.

A British fifty-pence coin has seven corners, and it still rolls. Its sides aren’t straight: each one is an arc drawn from the corner opposite, which makes the coin a curve of constant width. Measure it across in any direction and you get the same number, and that was the point when it arrived in 1969, because a vending machine judges a coin by its width, and a coin that measured differently depending on how it fell into the slot would be rejected half the time. There are infinitely many shapes like that, and a circle is only the plainest of them. What the circle has that the others don’t is a middle that stays put. Roll a fifty-pence piece between two rulers and they glide along perfectly flat, but its centre wobbles up and down as it goes, which is why nobody has ever put one on an axle. A wheel needs more than constant width. It needs a centre that stays where it is.

How many times should you shuffle a deck of cards? In 1992 two mathematicians, Dave Bayer and Persi Diaconis, worked it out for the ordinary riffle shuffle, where you split the deck in two and let the halves fall together: about seven. With fewer, the deck still carries its old order in long runs of cards that have only been interleaved, never really separated, and a careful player or a computer can find them. Past seven, more shuffling hardly helps. What I like is how sudden it is. The old order doesn’t fade evenly; it holds on almost untouched for four or five shuffles and then falls away over the next two or three. Diaconis had left home at fourteen to travel with a magician, which may be why he thought to ask. Most people shuffle three or four times, so most hands are dealt from a deck that still remembers the last game.

A crowded pan is why the mushrooms came out grey. Browning is the Maillard reaction, named for Louis-Camille Maillard, who described it in 1912: sugars and amino acids breaking apart and recombining into hundreds of new flavour molecules. It only gets going in earnest somewhere above 140 degrees Celsius. But a wet surface can’t get hotter than the water on it, and water boils off at 100. So everything depends on getting rid of the water faster than the food gives it up. Spread out, the steam escapes, the surface dries, and the temperature climbs past the threshold. Piled in, the pieces steam each other, and the pan spends all its heat boiling water at a hundred degrees while nothing browns. Mushrooms, which are about nine-tenths water, make the point better than anything. Patting things dry and cooking in batches sounds like fussiness, but it is really just arithmetic about where the heat goes.

Stack twelve perfect fifths on a low C and you ought to land on a C seven octaves up. You don’t quite. Twelve fifths overshoot seven octaves by about a quarter of a semitone, a gap called the Pythagorean comma, and no amount of careful tuning will close it, because it lives in the arithmetic: three-halves to the twelfth power is about 129.7, and two to the seventh is 128. A modern piano handles it by spreading the error evenly. Every fifth is tuned about two hundredths of a semitone flat, too little for most ears to catch, and in exchange every key works equally well. The major thirds pay more, about a seventh of a semitone sharp each, which is part of why a good choir singing on its own can settle into a sweeter chord than a piano can play. Every piano you have ever heard is slightly out of tune on purpose, so that it can be out of tune in the same way everywhere.

Around 1640 a Flemish physician named Jan Baptist van Helmont planted a five-pound willow in two hundred pounds of oven-dried earth, covered the pot to keep the dust out, and gave it nothing but water for five years. The willow came out weighing about 169 pounds. He dried and weighed the earth again and found it had lost only about two ounces. So the tree had not been made of soil, which was a good result, and he concluded it had been made of water, which was mostly wrong. Most of a tree’s dry weight is carbon, and the carbon comes out of the air, taken in through the leaves as carbon dioxide. The answer was the one thing in his experiment he had no way to weigh. When a log burns, most of it goes back the way it came: up, as the gas it was built from, leaving a little ash, the part that really did come from the ground.

The sunlight on a windowsill is about eight minutes old, which is the part everyone knows. The rest of its journey took far longer. Energy made in the Sun’s core can’t travel straight out: through most of the interior the gas is so dense that light is absorbed and given off again after a tiny step, in a random direction, over and over, so it staggers outward like someone lost in a crowd. Estimates of how long that walk takes run from about ten thousand years to well over a hundred thousand. Near the top, boiling gas carries the energy the rest of the way, and at the surface the light finally goes free and crosses the 150 million kilometres to us in about the time it takes to hard-boil an egg. Almost all of the trip happens where nobody can see it. The eight minutes is just the doorstep.

For about two thousand years purple was the colour of emperors, because the good purple came from sea snails, crushed by the thousands for a single gram of dye. In 1856 an eighteen-year-old named William Perkin, home for the Easter holidays, was trying to make quinine out of coal tar and got a black sludge instead. Cleaning out the flask with alcohol, he noticed the rinse had turned a strong purple, and he didn’t pour it away. Within a few years London was wearing mauve, made from the leftovers of the gasworks, and the colour of emperors was on ordinary people’s ribbons. I like that the find was in the washing-up: the experiment had already failed, and he was still looking.

An oak barrel that has stood empty and dry will usually leak when it is first filled again. It is held together by nothing but its shape: curved staves pressed edge to edge under hoops, no glue, no nails, and dry wood shrinks just enough to open the joints. Coopers don’t take it apart. They fill it with water and wait, because oak drinks, and the staves swell into their own gaps until the barrel holds. Winemakers call this swelling a barrel. I like a thing whose cure for not being ready is to be used anyway.

Almost nobody likes hearing their own voice on a recording, and there is a plain reason for it. When you speak you hear yourself two ways at once: through the air, like everyone else, and through the bones of your own skull, which carry the low notes especially well. A recording gives you only the air, so it sounds thinner and higher than the voice you have lived inside all your life. That means the voice you wince at is the closer likeness. It is the one your friends have been hearing all along, and they kept calling anyway.

Take any four-digit number whose digits aren’t all the same. Arrange its digits from largest to smallest, then smallest to largest, and subtract. Do it again with the answer, and again. Within seven steps you will land on 6174, and stay there, because 7641 minus 1467 is 6174. This year does it in six: 2026, 5994, 5355, 1998, 8082, 8532, 6174. A schoolteacher named D. R. Kaprekar found this in 1949, and for years hardly anyone took his little discoveries seriously. I like it because it isn’t useful for anything. It’s just a place every four-digit number is secretly walking toward.

A falling raindrop is never shaped like a raindrop. The small ones are round, pulled tight by surface tension. Bigger ones fall fast enough that the air pushes up on their undersides and flattens them, so a heavy drop comes down looking more like a hamburger bun than a tear. Past about four and a half millimetres the air wins: the underside caves in, the drop balloons into a thin bag, and it bursts into smaller drops, which is why rain comes in many sizes but never in large ones. The pointed shape in the forecast icons is closer to a drop hanging from a tap, just before it lets go.

A common swift that leaves its nest in late summer may not touch anything solid again until the next spring. When researchers in Sweden fitted small loggers to swifts, some of the birds flew for ten months without landing once. They eat insects on the wing, drink by skimming a lake in flight, and are thought to sleep up there too. Their legs are so short that the genus is called Apus, Greek for “footless”. Everything else a life needs, a swift does in the air. It comes down only to build a nest.