A For most of their history, the small mechanical devices that people used to tell the time were not worn on the wrist at all. From the sixteenth century onwards, the pocket watch was the standard. Kept on a chain inside a waistcoat, it was carried almost exclusively by men, and a fine one was less a practical tool than a mark of wealth and taste. Women, when they owned a watch, tended to wear a tiny version hung from a necklace or pinned to a dress, more ornament than instrument.

B When the first watches designed to be strapped to the wrist appeared in the nineteenth century, men largely refused to wear them. A watch on the wrist was widely regarded as a delicate piece of jewellery suitable only for women; a serious gentleman kept his timepiece in his pocket. This prejudice was strong enough that early manufacturers marketed wristwatches almost entirely to female customers, and few men would have been seen wearing one in public.

C What changed men's minds was not fashion but war. On the battlefield, an officer who had to pull a watch from his pocket, open its cover and check the time was at a serious disadvantage: co-ordinating an attack demanded that both hands stay free and that the time be read in an instant. During the colonial campaigns of the late nineteenth century, soldiers began improvising, strapping their pocket watches to their wrists with leather bands so that a glance was all that a synchronised advance required.

D The First World War completed the transformation. In the trenches, the wristwatch was no longer a curiosity but standard equipment, issued to officers and prized by ordinary soldiers. Manufacturers responded with rugged "trench watches", protected by metal grilles over the glass and fitted with dials coated in a glowing compound so that the time could be read at night without a lamp. A man returning from the front wearing a watch on his wrist was now admired rather than mocked, and civilian demand followed.

E For the next half-century the mechanical wristwatch reigned, until a very different threat arrived — not from a rival country's armies but from its factories. In 1969 a Japanese company introduced the first commercial quartz watch, kept in time by the tiny vibrations of a quartz crystal. Quartz movements were far more accurate than the finest mechanical ones and, once mass-produced, far cheaper. The flood of inexpensive, precise watches that followed devastated the traditional Swiss industry, an upheaval still remembered as the "quartz crisis".

F The most recent chapter is the smartwatch, a wrist-worn computer that counts steps, relays messages and, almost incidentally, shows the time. Yet the mechanical watch has not vanished. Freed from the job of being the most accurate option, it has been reborn as a luxury object, valued for its craftsmanship and history rather than its precision — proof that a technology made obsolete can still survive by changing what it is for.

A On a hot summer afternoon, a large city can be several degrees warmer than the countryside that surrounds it. This effect, which scientists call the "urban heat island", occurs because the concrete, brick and asphalt that cities are built from absorb the sun's energy during the day and release it slowly through the night, so the streets never fully cool down. Dark surfaces, a shortage of shade and the heat pouring from vehicles and air conditioners all add to the problem.

B The consequences are more serious than mere discomfort. During heatwaves, the extra warmth trapped in city centres raises the risk of illness and death, particularly among the elderly, and drives up the demand for electricity as millions of air conditioners switch on at once. Because that electricity is often generated by burning fuel, the machines that keep individual rooms cool end up making the wider city hotter still — a cycle that planners are increasingly anxious to break.

C One of the simplest remedies is also one of the oldest: plants. A street lined with mature trees can be noticeably cooler than a bare one, partly because the leaves provide shade and partly through a process called transpiration, in which the plant releases water vapour that carries heat away as it evaporates. Parks and gardens act in the same way, and a scattering of small green spaces spread across a district cools it more evenly than a single large park in one corner.

D A second approach targets the roofs. A "green roof", planted with grasses or low shrubs, insulates the building beneath it and cools the surrounding air, much as a garden does at street level. Its main disadvantage is cost: green roofs are heavy, need waterproofing and require regular care, which puts many owners off. A cheaper alternative is simply to paint a roof white. A pale surface reflects sunlight instead of soaking it up, and although it does nothing for the air quality that plants improve, it lowers indoor temperatures at a fraction of the price.

E Some cities have gone further and rewritten their rules. Singapore, often held up as the leading example, now requires many new developments to replace any greenery lost at ground level with plants on walls, balconies and rooftops, and offers grants to encourage them. The result is a skyline draped in vegetation that has measurably reduced temperatures in the built-up centre.

F No single method is a complete answer. The coolest cities of the future are likely to combine several — shade trees along the streets, reflective or planted roofs above, lighter-coloured pavements below — each contributing a small reduction that adds up. What is clear is that the way a city is designed, not just the weather it happens to receive, decides how hot its residents must endure.

A Few institutions divide opinion as sharply as the zoo. To their critics, zoos are relics of a less enlightened age, places where wild animals are confined for human amusement. To their defenders, the modern zoo is something quite different from the menagerie of a century ago: a centre for conservation, research and education that many endangered species can no longer do without. Deciding which view is closer to the truth requires looking past the emotion the subject arouses.

B The strongest argument in the zoos' favour is captive breeding. A number of species that had vanished from the wild — among them the Arabian oryx and the golden lion tamarin — survive today only because zoos maintained breeding populations and later released their descendants into protected habitats. Supporters argue that without such programmes these animals would already be extinct, and that no amount of protest could have saved them in the wild once their natural homes had been destroyed.

C Critics counter that these successes, though real, are rare. The overwhelming majority of animals in zoos belong to species that are in no danger at all, kept because they draw visitors rather than because they need saving. A breeding programme that returns animals to the wild, they point out, is expensive and difficult, and only a small fraction of zoos run one seriously. For every oryx restored to the desert, they say, there are thousands of creatures living out their lives purely on display.

D There is also the question of welfare. Even a spacious modern enclosure is a tiny fraction of the range an animal would cover in the wild, and for wide-ranging species such as elephants and big cats the difference can cause visible distress — repetitive pacing, poor health and shortened lives. Defenders reply that standards have risen dramatically, that good zoos now design enclosures around an animal's needs rather than the visitor's view, and that conditions vary so widely that sweeping judgements are unfair.

E Perhaps the least examined claim is the educational one. Zoos routinely describe themselves as places that teach the public to value wildlife, and it is often assumed that a child who meets a tiger will grow up to protect it. Yet the evidence that a zoo visit actually changes long-term behaviour is surprisingly thin, and some researchers suspect that the entertainment on offer overshadows any lasting lesson. This does not mean zoos teach nothing; it means the claim deserves more scrutiny than it usually receives.

F The honest conclusion is that the word "zoo" now covers institutions so different that a single verdict is impossible. A well-funded conservation zoo running genuine breeding programmes and a run-down roadside collection charging for photographs have almost nothing in common beyond the animals behind the glass. The useful question is therefore not whether zoos should exist, but which zoos deserve to.