A For thousands of years, money meant metal. Coins of gold, silver and bronze were valued because the material they were made of was itself worth something, and because that material was hard to fake. The trouble was weight. A merchant settling a large debt might need to transport a small fortune in heavy coin across dangerous roads, and it was this very inconvenience that eventually gave rise to a lighter idea.

B The solution first appeared in China. As early as the ninth century, traders who did not wish to carry strings of heavy copper coins began leaving them with trusted merchants, who issued a written note recording the amount deposited. The note could then be exchanged for the coins elsewhere, or simply handed on to settle a purchase. By the eleventh century the government had taken over the practice and was printing the world's first official paper currency, backed by the coins held in its treasuries.

C When European travellers such as Marco Polo described this Chinese "paper that circulated like coin", their readers found it almost unbelievable. Money, surely, had to be made of something valuable. It would be centuries before Europe adopted the idea, and when it did, the first notes were issued not by governments but by private banks. A banknote was in effect a promise: the bank guaranteed to pay the bearer, on demand, an equivalent amount in gold or silver held in its vaults.

D This link between paper and precious metal — the so-called gold standard — reassured the public that a note was as good as the coin behind it. But it also limited how much money could exist, since a country could print only as much as its gold reserves would cover. During major wars and economic crises, when governments needed to spend far more than their reserves allowed, the link became impossible to maintain.

E One by one, through the twentieth century, countries cut the tie. Today almost all money is what economists call "fiat" currency: it can no longer be exchanged for gold, and its value rests entirely on public confidence and the authority of the state that issues it. A modern banknote is worth what it is worth simply because everyone agrees to accept it — a remarkable act of collective trust.

F The story may now be entering its final chapter. In many countries physical cash is used less and less, replaced by cards and phones that move numbers between accounts without any note or coin changing hands at all. Some economists predict that paper money, having replaced metal a thousand years ago, will itself become a curiosity — outlasted by the very idea it introduced, that money need be no more than a trusted record of value.

A Every year, billions of birds travel thousands of kilometres between their breeding and wintering grounds, often returning to the very same field or hedge they left months before. How they manage such precise journeys, sometimes over open ocean where there are no landmarks at all, is a question that has puzzled scientists for well over a century. The answer, it now seems, is that birds do not rely on a single method but carry several different compasses at once.

B The most obvious guide is the sun. Many birds can tell direction from the sun's position, but since the sun moves across the sky through the day, this is only useful if the bird also knows the time. Experiments have shown that migrating birds possess an accurate internal clock and can combine it with the sun's position to work out true direction — a "sun compass" that adjusts automatically as the hours pass.

C Night-flying species face a different challenge, and they solve it with the stars. In a classic experiment, a researcher placed young birds inside a planetarium and slowly rotated the artificial night sky. The birds oriented themselves not by any single star but by the pattern of rotation around the celestial pole — the fixed point in the sky about which the stars appear to turn. Remarkably, they seemed to learn this pattern while still in the nest, by watching the real sky.

D The most surprising discovery is that birds can sense the Earth's magnetic field. This "magnetic compass" works even in total darkness or heavy cloud, when neither sun nor stars are visible, and it appears to tell the bird not east or west but whether it is heading towards or away from the poles. Exactly how the sense works is still debated, but it gives migrants a reliable backup when the sky is hidden.

E Closer to home, birds fall back on landmarks — coastlines, rivers, mountain ranges and, increasingly, human features such as motorways. Some species also appear to use smell, building a kind of odour map of their home region. None of these methods alone would be enough for a journey of thousands of kilometres, but together, cross-checked against one another, they add up to a navigation system of astonishing reliability.

F Understanding all this is more than a matter of curiosity. As human activity reshapes the planet — with artificial light that drowns out the stars, structures that birds collide with, and a climate that shifts the timing of the seasons — knowing how birds navigate is becoming essential to protecting the great migrations before they are disrupted beyond repair.

A A growing number of cities, from Oslo to Bogotá, have begun removing private cars from their central districts, whether permanently or on particular days. The idea provokes strong reactions. Supporters see it as an overdue correction to a century in which the car was allowed to dominate urban space; opponents warn of damage to businesses and hardship for those with no easy alternative. As with most such debates, the evidence is more mixed than either camp admits.

B The case for closure rests first on health. Traffic is a major source of the air pollution that shortens lives in almost every large city, and cars are also the main producer of the noise that residents most often complain about. Where through-traffic has been removed, measurements typically show cleaner air and quieter streets within weeks. Advocates add that streets freed from parked and moving cars can be returned to people — for markets, cafés, play and simply walking — reviving a kind of public life that heavy traffic had driven away.

C Retailers are often the loudest objectors, fearing that customers who cannot drive to the door will shop elsewhere. Yet the economic evidence tends to cut the other way. Studies of pedestrianised streets repeatedly find that shopkeepers overestimate how many of their customers arrive by car and underestimate how much people on foot or bicycle spend over time. In several cities, retail takings rose after cars were removed, not fell — though the effect varies, and a poorly planned scheme can indeed drive trade away.

D The strongest objection is fairness. Not everyone can walk or cycle, and not every journey can be made by public transport. For elderly or disabled residents, for parents with small children, and for tradespeople carrying tools, a car may be a necessity rather than a luxury. Critics argue that car-free schemes designed by and for the fit and affluent can quietly exclude those who most depend on driving, unless careful exceptions and good alternatives are built in from the start.

E This points to what the successful examples have in common. Cities that have made car-free centres work did not simply ban vehicles; they invested first in the alternatives — frequent public transport, safe cycle routes, and delivery and access arrangements for those who genuinely need them. Where such groundwork was skipped, restrictions have often had to be reversed in the face of public anger.

F The lesson, then, is not that cities should or should not close their centres to cars, but that how they do it decides whether it succeeds. A ban imposed overnight without alternatives is a very different thing from one introduced gradually alongside real investment in other ways of getting around. The question worth asking is not whether the car should give up some of the city, but on what terms.