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A In the coastal deserts of northern Chile, where rainfall can be measured in millimetres per year, an unlikely source of water hangs in the air for much of the morning. Thick banks of fog, driven inland by Pacific winds, drift across the hills before evaporating in the midday sun. For most of the twentieth century, this moisture was considered a meteorological curiosity. Today, it is the basis of a growing technology known as fog harvesting.

B The principle is disarmingly simple. A vertical mesh, often made of polypropylene, is suspended between two poles in the path of incoming fog. As droplets strike the mesh, they coalesce and run down into a gutter, then into a storage tank. A single well-placed net of around forty square metres can yield between 150 and 300 litres of water on a good day — enough to supply a small household.

C Although the modern technology was developed in the 1980s by Canadian physicist Robert Schemenauer and Chilean colleagues, the idea is far older. Indigenous communities in the Atacama placed cloth and animal skins on rocks to collect dew, and in the Canary Islands, the so-called "fountain trees" of El Hierro — large laurels whose leaves dripped condensed mist — supplied entire villages well into the eighteenth century.

D The appeal of fog harvesting is its low cost and its independence from energy infrastructure. Unlike desalination, which requires substantial electrical input, fog nets work passively. Maintenance is minimal: most damage comes from high winds rather than mechanical wear. For isolated mountain communities, the cost per litre of harvested water is typically a fraction of what trucked-in supplies cost.

E Yet the technology is not without problems. Fog is geographically restricted; it requires the right combination of cool ocean currents, prevailing onshore winds, and elevated terrain to condense reliably. Even in suitable regions, output varies dramatically from week to week. In one well-documented project in Guatemala, monthly yields fluctuated by a factor of eight. This unpredictability makes fog water a useful supplement rather than a primary source for most users.

F Recent research has focused on improving collection efficiency. Conventional polypropylene mesh captures only about 2% of the water present in passing fog; most droplets either bounce off or are deflected by the air flow around the mesh. Researchers at MIT have developed coatings that combine hydrophilic and hydrophobic patches, mimicking the surface of the Namib desert beetle, which collects fog on its back. Field trials of these surfaces show capture rates of up to 10%.

G Critics, however, caution against framing fog harvesting as a stand-alone solution. Hydrologist Maria Soto, who has studied water-scarce communities in Peru, argues that the most successful projects are those embedded in broader strategies — including rainwater capture, greywater reuse, and community-managed distribution. Where fog projects have failed, she notes, the cause has usually not been the technology itself but a lack of long-term local stewardship.

H Looking ahead, the global potential of fog harvesting remains modest in absolute terms. It cannot, and is not intended to, supply cities. But for communities living in arid uplands above productive cloud belts, it offers something rarer than abundance: a reliable, low-energy supplement to a brittle water supply, and one that builds on knowledge their ancestors already understood.

Yawning is one of the most universal of human behaviours and one of the least understood. We yawn in the womb, we yawn when tired, we yawn when bored, and — most curiously — we yawn when we see other people yawn. Despite more than a century of research, no single explanation accounts for all of this.

The oldest theory, dating from the 19th century, held that yawning increases the level of oxygen in the blood. The idea was intuitive: a deep, gaping inhalation should, surely, draw in more air. But controlled studies in the 1980s by physiologist Robert Provine showed no such effect. People asked to breathe pure oxygen did not yawn less, and people made to breathe air with elevated carbon dioxide did not yawn more. The oxygenation hypothesis, simply, did not survive the evidence.

A more promising current theory is that yawning cools the brain. The act of yawning involves a powerful stretch of the jaw muscles and a sharp intake of cool air across the sinus cavities. Both of these, in principle, could lower the temperature of blood flowing to the brain. Andrew Gallup, an early proponent of this view, found in 2010 that volunteers yawned more often when ambient temperatures rose to 22°C than when they were either much colder or much warmer. Above a certain threshold — around 37°C — yawning frequency dropped sharply, presumably because outside air no longer offers a cooling effect.

The thermoregulation hypothesis also offers an explanation for why we yawn more when tired. Brain temperature rises modestly during periods of sleep deprivation and falls during deep sleep. A yawn before bed, on this view, is a small physiological gesture to bring brain temperature back into its preferred range.

But yawning has another, stranger property: it is contagious. Roughly 60% of adults will yawn within five minutes of watching another person yawn, and the rate is similar even if the yawn appears only in text on a page, or in a still photograph. Children begin to "catch" yawns at around four years of age. Below that age, contagion is rare. Contagious yawning has also been documented in chimpanzees, in some dogs, and tentatively in elephants — but not in solitary species.

This distribution has led some researchers to propose that contagious yawning is a form of low-level empathy. Catriona Morrison and colleagues at the University of Leeds reported in 2011 that adults who scored higher on empathy questionnaires were significantly more likely to "catch" yawns from others. People diagnosed with conditions associated with reduced cognitive empathy, including some on the autism spectrum, were less susceptible. This is suggestive rather than conclusive: the effect sizes are modest, and other studies have failed to replicate them.

An alternative view is that contagious yawning serves a synchronising function within social groups. In wolves, observed yawning spreads most quickly between animals with the closest bonds, regardless of dominance. Behavioural ecologist Elisabetta Palagi has suggested that yawn contagion may help align group activity — that is, group members slipping toward a shared state of rest, alertness, or transition.

What no one disputes is how strangely difficult yawning is to study. It cannot be reliably triggered on demand, it occurs both spontaneously and socially, and it resists tidy categorisation as either reflex or emotion. For a behaviour we perform thousands of times in a lifetime, it has been remarkably good at keeping its secrets.

The thermoregulation hypothesis proposes that yawning helps to cool the . Gallup observed that yawning peaked at around °C, but became rare when ambient temperatures rose above roughly °C. The same hypothesis also accounts for the increase in yawning during periods of .

By the middle of this century, around two-thirds of the world's population will live in cities, and a remarkable number of them will live in cities of more than ten million inhabitants — the so-called megacities. Lagos, Dhaka, Kinshasa and Karachi are already expected to double in size within a generation. Whether their administrative systems can keep pace is far less certain.

Megacities are not simply enlarged versions of smaller cities; they are functionally different. As population doubles, the network of physical and institutional connections does not double — it expands at a faster rate, producing not only more demand but more complexity per inhabitant. Sewerage, road traffic, electricity distribution and health systems become harder to model, harder to repair, and harder to govern. This phenomenon, sometimes termed "superlinear scaling", was first quantified by physicist Luís Bettencourt and his colleagues, who found that economic output and innovation in cities tend to rise more than proportionately with size — but so do crime, congestion and the cost of services.

Most national governments were not designed with such cities in mind. Many of the world's largest urban areas span multiple municipalities, sometimes multiple states or provinces. Greater Tokyo, for instance, comprises 23 wards plus dozens of cities and towns, while São Paulo's metropolitan region contains 39 separate municipal authorities. Coordination between these bodies is voluntary in most jurisdictions, and competition for tax revenue often undermines joint planning.

The result, urbanists argue, is a chronic gap between the scale of urban problems and the scale of urban institutions. A megacity may have hundreds of agencies handling housing, transport, health and waste, but no single body with authority to direct them. In a study of 27 megacities, the LSE Cities programme found that fewer than half had a single metropolitan-level government with binding authority; in most cases coordination depended on informal agreements that broke down under fiscal pressure.

This institutional fragmentation matters most during shocks. The COVID-19 pandemic, for example, revealed striking differences in how megacities responded. Seoul, which has an empowered metropolitan government and an integrated data system across districts, was able to roll out testing and contact tracing as a coherent operation. By contrast, cities where authority was split across competing bodies often pursued contradictory policies in adjacent neighbourhoods. The pandemic, in this sense, was a stress test that the most fragmented cities failed.

Some scholars, most notably the political scientist Benjamin Barber, have argued that cities should be granted formal recognition in international affairs. Barber's If Mayors Ruled the World proposed a "Global Parliament of Mayors", on the grounds that megacity leaders are increasingly closer to global problems — pandemics, migration, climate adaptation — than the national governments under whose flags they nominally operate. Critics dismiss the idea as utopian: cities, they note, lack the legal sovereignty and military capacity of states. Yet a quieter version of Barber's vision is already emerging in networks such as C40 Cities, which coordinates climate policy among the world's largest urban centres without any formal treaty.

What remains unresolved is whether megacities can be made governable without sacrificing the very thing that makes them productive. Their concentration of people and capital generates extraordinary creative density; the same density also overwhelms administrative routine. To centralise power risks suppressing local responsiveness; to fragment it risks paralysis. The cities themselves are unlikely to wait for theorists to settle the question. In the meantime, the more pragmatic of their mayors are building parallel institutions — data offices, regional coordination councils, public-private partnerships — as workarounds for the structures they have inherited.

Whether these workarounds will prove sufficient, or whether some future generation will have to design wholly new forms of urban government, is the central political question of urbanisation in the present century.

A. Quiet Study Zone (3rd floor) — Mobile phones must be silenced. Group discussion is not permitted. Children under 12 are not allowed in this area, even with an adult.

B. Children's Library (1st floor) — Open every day from 9 a.m. Story time on Wednesdays and Saturdays at 11 a.m. Adults may borrow children's books only with a junior reader.

C. Computer Suite (2nd floor) — Free for members, 90 minutes per day. Printing 10p per page (black & white) or 50p (colour). Book ahead online; walk-ins available if free machines.

D. Café (Ground Floor) — Hot drinks and snacks. Closes 30 minutes before the library. Reusable cups receive a 20p discount. No hot food may be taken upstairs.

E. Returns & Renewals — Items can be returned 24/7 to the outside drop-box. Loans can be renewed twice unless reserved by another reader. Late return: 15p per item per day, capped at £5.

Welcome to Linden Court. Please keep this sheet for reference.

Rent and bills. Rent is due on the 1st of each month by bank transfer. Late payment after 7 days incurs a £25 administration charge. Electricity, gas and council tax are paid directly by tenants; water is included in the rent.

Repairs. Report any urgent repair (no heating, leaking pipe, broken lock) by phone on 0117 555 9020 at any hour. Non-urgent issues should be submitted via the online form; the agency aims to respond within five working days.

Decoration. Tenants may hang pictures using removable adhesive strips only. Painting or wallpapering is not permitted without written consent. Holes in plaster will be charged at end of tenancy.

Visitors. Overnight guests are welcome but must not stay for more than fourteen consecutive nights. Subletting any part of the flat — including via short-term rental websites — is strictly forbidden and will lead to immediate termination of the tenancy.

Moving out. One full month's written notice is required. The flat must be returned in the condition shown in the inventory, allowing for fair wear and tear. The deposit is held in a government-approved scheme and is returned within 10 working days of an agreed final inspection.

Eligible staff may work from home up to two days per week, subject to the agreement of their line manager. New employees must complete their six-month probation period before applying.

On home-working days, staff are expected to be available by phone and on the company messaging platform during their normal working hours, with at most a 30-minute response delay. Cameras must be turned on during scheduled meetings. Recording any meeting requires the consent of all participants.

The company will provide a chair, a second monitor and a headset on request — but not a laptop, which all staff already receive on joining. Equipment must be returned in working order when employment ends. Reasonable home-broadband costs (currently £20 per month) are reimbursed automatically on submission of a single bill at the start of each tax year.

Confidential documents may not be printed at home unless a shredder is available, and may not be discussed in shared living areas where non-staff can overhear. Any data breach, however minor, must be reported to the line manager within 24 hours.

The policy will be reviewed annually. Suggestions can be sent to the People & Culture team at any time.

A. Annual leave. Requests for July or August must be submitted by 1 May. Requests received later cannot be guaranteed and will be considered on a first-come basis.

B. Cycle-to-work scheme. Staff can now buy a bicycle through the company and pay back the cost in monthly instalments deducted from salary, saving up to 32% on tax. Open to anyone who has passed probation.

C. First aid. Two new first aiders have been trained on the second floor. Please learn their names: Pavla Novak and Eshan Reddy. A full list of first aiders is posted by every printer.

D. Lift maintenance. The main lift will be out of service from Saturday 14th to Monday 16th of this month. Staff requiring step-free access during this period should email facilities@ in advance so alternative arrangements can be made.

E. Coffee machine. The third-floor machine is being replaced. Until the new one arrives, please use the machines on the second or fourth floor. Mugs should not be left in the sink overnight.

The slow rise of community gardens

Community gardens — shared plots of urban land, cultivated collectively by local residents — are sometimes presented as a recent invention. In fact, the practice has a long and uneven history. The first formally organised community gardens in industrial cities appeared in late 19th-century Germany, where so-called Schrebergärten were created on the edges of crowded towns to give working families access to fresh air and home-grown vegetables. Similar gardens spread to Britain, France and the United States within a generation.

The first major boom came during the World Wars. "Victory gardens" were promoted by several governments as a patriotic way to ease pressure on national food supplies. At their peak, such gardens are estimated to have produced almost forty per cent of the fresh vegetables consumed in some American cities. After 1945, however, interest faded quickly. Cheaper supermarket produce, rising car ownership and suburbanisation made small-scale urban growing seem old-fashioned.

A second wave appeared, more quietly, in the 1970s, often led by activists in inner-city neighbourhoods that had been damaged by housing demolition or industrial decline. The "Green Guerillas" of New York, founded in 1973, planted seedbombs over abandoned lots, transforming several into thriving gardens that survive to this day. Similar initiatives took root in London, Berlin and Detroit, almost always on land considered marginal or worthless by official planners.

Today, the appeal of community gardens is unusually broad. For some participants, the motivation is environmental: a cooler microclimate, a habitat for pollinators, a way to reduce the heat-island effect that affects dense neighbourhoods in summer. For others, the draw is social. Surveys of gardeners in cities as varied as Toronto, Lisbon and Seoul consistently report increased contact with neighbours and a stronger sense of belonging to the local area. For older gardeners in particular, the activity offers a structured reason to leave the house and meet others, with measurable effects on reported loneliness.

Health is another commonly cited benefit, though the evidence is mixed. Studies in the UK and Netherlands have found that regular gardeners report better mental health than matched non-gardeners, and have somewhat lower body mass indexes. Whether the garden itself causes these effects, or whether healthier people are simply more likely to take up gardening, is harder to establish. The most careful longitudinal studies suggest both directions of influence are real, but neither effect is large.

There are, of course, difficulties. Land tenure is the most persistent problem. Many community gardens occupy plots that belong to local authorities, charities or private owners, and almost none are protected indefinitely. When property prices rise, the gardens are often the first thing to disappear; in some cities, a thriving garden of fifteen years has been bulldozed within months once a developer's offer has been accepted. Even where land is secure, recruitment can be uneven, water access may be poor, and conflicts over rules, work-sharing and the use of pesticides recur in almost every group.

What makes the most successful gardens last is not, as is sometimes assumed, an especially talented coordinator or a single generous patron. Comparative studies — including a recent ten-city review by the urban planner Yusra Khalil — point instead to less visible factors: a written agreement with the landowner, a clear and accepted procedure for resolving disputes, and a steady habit of welcoming newcomers, particularly those from groups not usually associated with gardening. Where these are in place, gardens often outlast the initial enthusiasts by decades. Where they are missing, even the most productive plot can collapse within a single growing season.

During the World Wars, so-called were promoted by governments to support national food supplies. After 1945, interest declined as cheaper supermarket produce and spread. A second wave emerged in the 1970s, often on land regarded by official planners as . In dense neighbourhoods, the gardens are valued partly because they reduce the in summer.