A Sending a letter today is so simple that we rarely think about it: we buy a stamp, stick it on, and post it. Yet before 1840, the system was very different, and far more troublesome. In most countries it was the person who received a letter, not the person who sent it, who had to pay. The charge depended on how far the letter had travelled and on the number of sheets of paper it contained. A long letter sent across the country could cost more than a worker earned in a day, and many people simply refused to accept letters they could not afford.

B The man who changed all this was an English teacher and reformer named Rowland Hill. He argued that the sender, not the receiver, should pay, and that everyone should pay in advance. Even more radically, he proposed a single flat rate — one penny — for any letter under a certain weight, no matter how far it was going. To show that payment had been made, a small piece of printed paper would be stuck to the envelope. In 1840 the first such adhesive stamp, the famous "Penny Black", went on sale. It carried a portrait of the young Queen Victoria.

C The effect was immediate and enormous. Because posting a letter was now cheap and simple, the number of letters sent in Britain more than doubled within a year. The idea spread quickly abroad, and within a decade many other countries had issued adhesive stamps of their own. Stamps soon became more than a proof of payment: with their pictures of monarchs, landscapes and national symbols, they turned into tiny works of art that a country could show to the world.

D The earliest stamps did have one awkward flaw. They were printed in sheets with no lines between them, so post office clerks had to cut each stamp from the sheet with scissors — slow work that often left the stamps with crooked edges. The problem was solved in 1854, when perforations — the rows of small holes we still see today — were introduced, allowing stamps to be torn apart neatly by hand. By then, collecting stamps had already become a popular hobby, one that continues to fascinate millions of people around the world.

A For hundreds of years, the beaver was a familiar animal in Britain's rivers. By the sixteenth century, however, it had been hunted to extinction across the country. People valued it for three things: its thick fur, its meat, and a substance from its body called castoreum, which was once used in medicine and perfume. For four centuries the beaver was gone — until, in recent decades, a series of carefully managed projects began to bring it back.

B Scientists were keen to reintroduce the beaver because of the remarkable effect it has on the landscape. Beavers are often called "ecosystem engineers" because, more than almost any other animal, they reshape the world around them. By felling small trees and building dams across streams, they create ponds and wetlands where there was once only fast-flowing water. These new habitats attract a wide range of wildlife: fish shelter in the deeper pools, dragonflies and other insects breed at the water's edge, frogs and newts thrive in the shallows, and birds arrive to feed on all of them.

C The benefits reach human communities too. Because a beaver dam slows the movement of water, it holds rainfall back high in a river system instead of letting it rush downstream all at once. This reduces the risk of flooding in towns further down the valley. The ponds also act as natural filters, trapping mud and pollutants so that cleaner water flows out the other side. And in periods of dry weather, the water stored behind the dams is slowly released, helping streams to keep flowing when they might otherwise run dry.

D Not everyone welcomes the beaver's return. Some farmers worry that dams will flood valuable fields or block the drainage channels that keep their land workable. These concerns are real, and in a few cases beavers have had to be moved. Yet the trials carried out so far suggest that, with sensible management, the advantages clearly outweigh the difficulties. Slowly but surely, the beaver is once again becoming part of the British countryside.

A Across the world, a growing number of cities are taking steps to reduce the number of private cars in their centres. Some charge drivers a fee to enter, a policy known as congestion charging; others create car-free zones or turn busy streets into pedestrian areas where only people on foot are allowed. Supporters of these measures argue that they bring clear benefits: cleaner air, quieter and safer streets, and more public space that can be given over to parks, cafés and cycle lanes rather than parking.

B Critics, however, raise serious objections. The most common is economic: if shoppers can no longer drive to a district, they claim, its shops and restaurants will lose customers and may be forced to close. There is also a question of fairness. Not everyone can easily manage without a car. People with certain disabilities, families living in the suburbs, and those in areas with poor public transport may all depend on driving in ways that a fit city-centre resident does not. To such people, restrictions can feel less like an environmental policy and more like a penalty.

C What does the evidence actually show? Here the picture is genuinely mixed. In some cities, pedestrianising a central street has been a clear success: with cars gone, the area became more pleasant, and the number of visitors — and the takings of local businesses — actually rose. In others, however, the same policy was followed by decline, as customers simply drove to out-of-town shopping centres instead. The difference, researchers increasingly agree, lies in what replaces the car. Where a city has invested in frequent, affordable buses, trams or trains, people adapt easily; where it has not, restricting cars only pushes activity elsewhere.

D This points to a sensible conclusion. Discouraging private cars is not, in itself, either good or bad; its success depends almost entirely on whether real alternatives are offered at the same time. A city that removes cars while giving people convenient, reliable ways to travel is likely to end up cleaner, safer and more prosperous. A city that simply bans cars and hopes for the best, without investing in transport, is likely to be disappointed — and to have punished the very residents it was meant to serve.