A Long before paper existed, people wrote on whatever materials they could find. In some places they carved words into clay tablets that were heavy to carry; in others they used papyrus, made from a river plant, which crumbled and cracked with age. In Europe, important documents were written on parchment, a fine writing surface made from animal skin — beautiful and long-lasting, but so slow to prepare that it was extremely expensive.

B Paper as we know it was invented in China. Tradition credits its creation to Cai Lun, an official at the imperial court, around the year 105 CE. He is said to have crushed together tree bark, cloth rags and old fishing nets, soaking the mixture in water until the fibres separated, then pressing it into thin, even sheets and leaving them to dry. The result was light, cheap and easy to write on.

C For several centuries China guarded the method as a closely kept secret, and paper spread only slowly beyond its borders. The craft is believed to have reached the Islamic world after a battle in the eighth century, when Chinese papermakers were taken prisoner and made to teach their skill. Soon afterwards the first paper mills in the region were built in the city of Baghdad, and from there the knowledge travelled steadily westward.

D Paper finally reached Europe through Spain and Italy. At first many people distrusted it, believing it to be far less durable than parchment, and a number of rulers even forbade its use for official records. But paper was so much cheaper to produce that its advantages could not be ignored. When the printing press arrived in the fifteenth century and books began to be produced in their thousands, cheap paper was no longer a curiosity but a necessity.

A More than half of the world's people now live in cities, and the share is still rising. As neighbourhoods grow denser and buildings taller, many residents find themselves living further than ever from open countryside. Precisely because nature has become harder to reach, the parks, gardens and tree-lined streets that survive within cities matter more than they once did.

B The clearest case for urban greenery is what it does for the people who live nearby. Time spent among trees and grass has been shown to lower stress, encourage walking and other exercise, and lift mood. In one well-known study, hospital patients whose windows looked out onto trees recovered more quickly than those who faced a brick wall. Access to a nearby park is now regarded by many doctors as a genuine public-health resource.

C Green spaces also change the physical climate of a city. Dark roofs and paved roads absorb the sun's heat and release it slowly, so that built-up areas can be several degrees warmer than the countryside around them — an effect known as the “urban heat island”. Trees cool the air as they release water vapour, and their shade keeps streets and buildings from heating up in the first place.

D A less obvious benefit appears when it rains. Concrete and tarmac cannot soak up water, so heavy rain runs straight off them and overwhelms the drains, causing floods. Parks, lawns and gardens act like a sponge: their soil absorbs rainwater and releases it gradually, easing the pressure on a city's drainage and reducing the risk of sudden flooding.

E Cities are home to more than people. Parks and gardens provide food and shelter for birds, bees, butterflies and countless other creatures, many of which struggle to survive in a landscape of glass and stone. Where strips of greenery are joined together, they form corridors that allow wildlife to move safely between otherwise isolated patches of habitat.

F Yet the green in our cities is not shared equally. Wealthier districts tend to have leafy streets and well-kept parks, while poorer neighbourhoods — where the benefits would matter most — often have very little. Aware of this gap, a growing number of city planners now set targets to ensure that every resident lives within a short walk of a green space, treating fair access to nature as a basic right rather than a luxury.

The idea that people could work fewer hours and yet get just as much done was, until recently, dismissed as fantasy. Surely, the argument went, less time at work must mean less work completed. Yet a growing number of companies, and even a few governments, have begun to test a four-day week — and the early results have been difficult to ignore.

Supporters insist that fewer hours need not mean lower output. When a large trial in Britain gave staff a paid day off each week without cutting their wages, most of the firms taking part reported that the amount of work produced stayed the same, and in some cases even rose. Freed from long and tiring weeks, employees seemed to waste less time and to concentrate more sharply on the hours they did spend at their desks.

The gains reached well beyond the office. Workers described lower levels of stress, better sleep and more time to spend with their families. Rates of sickness fell, and remarkably few employees said they would choose to return to a five-day week, even in exchange for higher pay. For many of them, the extra day of freedom was simply worth more than the money.

Critics, however, warn against treating the four-day week as a cure for every workplace. It fits office work far more easily than industries such as healthcare, transport or retail, where someone must be present at all times. Some economists add a further note of caution: the companies in these early trials had volunteered to take part and were eager to prove the idea a success, so the outcome might look rather different if every business were compelled to adopt it.

What almost everyone now accepts, though, is that the old assumption — that longer hours automatically produce greater output — no longer holds. Whether or not the four-day week eventually becomes the norm, it has already changed the way we think about the relationship between time and productivity.