A For most of human history, making fire was slow and uncertain work. People struck flint against steel to produce a spark, or kept a fire burning day and night so that it never had to be relit. The invention of the friction match in the nineteenth century changed daily life more than we might imagine: for the first time, a flame could be produced in a single second, by anyone, at almost no cost.

B The first friction matches were made in 1826 by John Walker, an English chemist. He had been mixing chemicals with a wooden stick when he noticed that the dried lump on the end burst into flame as he scraped it against the stone floor. Walker sold his matches in his local town but never patented them, and so he earned very little from an invention that would soon be found in almost every home.

C Early matches, however, were dangerous. Their heads were tipped with white phosphorus, a substance that caught fire easily but was also highly poisonous. The young women who worked in the match factories breathed in its fumes for hours each day, and many developed a painful and disfiguring disease of the jaw. Public anger at these conditions eventually grew into one of the first campaigns for factory safety.

D The solution came from Sweden. In the 1850s, manufacturers there developed the “safety match”, which separated the chemicals needed to start a fire. The match head itself no longer contained phosphorus; instead, the phosphorus was painted onto the side of the box. A match would now light only when struck against that special surface, making accidental fires far less likely. The design was so successful that it remains, essentially unchanged, in use today.

A Every year, millions of animals undertake journeys that would defeat any human traveller without a map. The Arctic tern flies from one pole to the other; young eels cross entire oceans to reach the river where they will breed. What is remarkable is not only the distance but the accuracy: many animals return to the exact spot where they were born, often having never made the journey before.

B One of the oldest known tools of animal navigation is the sun. Many birds use it as a compass, adjusting for the fact that it moves across the sky during the day. To do this they rely on an internal clock — a sense of time that tells them where the sun should be at any given hour. Cover a bird's view of the sun, and its sense of direction becomes far less reliable.

C Other animals carry a compass of a different kind. Sea turtles, salmon and even some bacteria can detect the Earth's magnetic field, using it to tell north from south. Newly hatched turtles appear to memorise the magnetic “signature” of their home beach, and use it decades later to return there and lay their own eggs. Exactly how they sense magnetism is still not fully understood.

D Smell and memory play their part too. Salmon are thought to recognise the particular chemical scent of the stream in which they hatched, following it upriver against the current. Homing pigeons, famously, combine several methods at once — sun, magnetism and familiar landmarks — which may be why they are so difficult to fool.

E For a long time scientists assumed that each species relied on a single method. It is now clear that most animals use several, switching between them as conditions change. On a cloudy day, when the sun is hidden, a bird may fall back on its magnetic sense; closer to home, it may rely on landmarks it remembers.

F Understanding these abilities is more than a scientific curiosity. As human activity fills the environment with artificial light and electromagnetic noise, some of these natural signals are being disrupted. Knowing how animals navigate may prove essential to protecting the great migrations before they are lost.

The question of whether money can buy happiness is one of the oldest in economics, and the answer, it turns out, is: partly. Research consistently shows that as people move out of poverty, their wellbeing rises sharply. Having enough to eat, a secure home and access to medical care makes an enormous difference to how satisfied people feel with their lives. Below a certain income, in other words, money and happiness are closely linked.

Above that level, the picture changes. Once basic needs are met, further increases in income bring smaller and smaller gains in happiness. A person whose salary doubles from a very low figure feels the change deeply; a wealthy person whose salary doubles again barely notices. Economists call this pattern “diminishing returns”, and it appears in study after study across many different countries.

Part of the explanation lies in comparison. People judge their income not in absolute terms but against those around them. A pay rise feels wonderful — until one discovers that a colleague received a larger one. Because we constantly compare ourselves with others, the satisfaction of earning more tends to fade as our expectations rise to match our new circumstances.

What does seem to bring lasting happiness is not income itself but how it is used. Studies suggest that spending money on experiences — a trip, a concert, an evening with friends — brings more durable satisfaction than buying possessions. Spending on other people, rather than on oneself, has a similar effect. The lesson is not that money is unimportant, but that beyond a point, what we do with it matters more than how much of it we have.