What is water? It is two atoms of hydrogen and one of oxygen, fused together. It is a building block of our chemistry, the determinant of the centigrade scale by which we measure temperature - its freezing point designated zero, its boiling point 100. It is a poor conductor, a good insulator; pour some on phosphorous and stand back. It is odourless, colourless, tasteless and calorie-less, neither acid nor alkali.
On the surface, water seems a benign substance. But deeper down, it has a strange allure. In a mass, moved by tides or gravity, water seems to have a life of its own. An elemental force of many moods, it has inspired all kinds of human endeavour from paintings to pop songs.
Water is in the ground and in our bodies, in the seas and in the skies. It rots our houses and floods our streets, feeds our crops and adorns our parks and gardens. Way back in evolution, we crawled out of it. Before we are born we spend nine months suspended in it. We may work on it, rest in it or play in it but most of all we depend on it. Because without water there is no life.
Last year, scientists claimed to have discovered water on the moon. Not very much - mere traces, deep-frozen below the dusty poles of the planet - but enough to make them very excited. Last Saturday NASA crashed a satellite on the moon in the hope that dust from the impact would reveal traces of water. But astronomers say the results are ambiguous and that it will be "some time before we know it there is water there".
These dreams are unlikely to be realised. Other planets have large deposits of ice - Europa, Jupiter's second satellite, is covered in it - but Earth is the only planet with large quantities of H2O in its liquid form.
We have oceans of the stuff - water covers 71 per cent of our planet's surface. In some countries it's abundant. But everywhere in the world, wet or dry, water is wasted. In the developed world, each person uses an average of 135 litres of water a day - more than 10 times as much as someone in the developing world. Many people have no more than two-and-a-half litres a day, for drinking, washing and cooking - about what we use in a single flush of the toilet.
Although we could all do more to save water, domestic users are not the main culprits. Overall, households consume 10 per cent of the United Kingdom's water supplies, industry 25 per cent and agriculture 65 per cent. The biggest users are also the biggest misusers of water. Inefficient irrigation means millions of gallons of water evaporate before they can be absorbed. Rivers, the natural irrigators of the land, are bled dry to feed thirsty crops.
Dr Norman Myers, an independent scientist who has advised the World Bank on water, says that without remedial action to use water more sensibly, the gap between the water-rich and the water-poor countries will continue to grow. "Parts of the world, such as Britain and north America will be fine. But Africa is traditionally dry and getting drier and China is in a lot of trouble."
The proportion of the world's population which is short of water is set to double over the next 25 years. "These are people who really are thirsty," Dr Myers says, ominously. By 2025 three billion people will be suffering from "water scarcity" and many millions more from "water stress".
The biggest threat to life is not lack of water (hardly anyone dies of thirst) but lack of clean water. The UN's World Commission on Water says that half the rivers and lakes in the world are seriously polluted. It estimates that 1.4 billion people live without clean drinking water and almost twice that number lack adequate sanitation. Seven million people die each year from water-borne diseases such as cholera, malaria, yellow fever and diarrhoea.
A large part of the problem, and a little known fact, is that governments routinely subsidise the cost of water. According to Dr Myers: "Almost all governments charge between a quarter and a fifth of what it costs to supply the water, which is crazy." If water users were made to pay a more realistic price, water would be used and recycled more efficiently, he says.
Is there such a thing as pure water? There's ordinary water - which, if you live in the developed world, comes out of a tap, and if you don't could involve a walk of several miles - and then there's mineral water, which comes in a bottle.
Mineral water has been one of the success stories of recent years. In the 1980s, when sales increased 20-fold, it was a marketing man's wet dream. Before that the only time most people bought bottled water was when they went abroad with the mantra - "don't drink the water" - ringing in their ears .
The pioneer was Perrier. With its bulbous green bottle and punning sl-eau-gons, Perrier's carefully cultivated image of fizzy sophistication soon made it a regular at dinner parties and in restuarants.
Then, in 1990, traces of the chemical benzene were found at Perrier's source in Vergeze. The quantities were so small that they posed no real health risks, but in a business so based on purity, Perrier's image was at stake and the company ceased production for six weeks.
British brands got a foothold in the market and supermarket shelves now groan beneath the variety of waters available. Last year we drank 945 million litres of bottled water.
Bottled water comes in three main varieties - table water (little more than purified water), spring water (which comes from a single underground source) and the pedigree brand - natural mineral water - which has to meet high standards of purity over a two-year period before it can carry the title.
Water power The Industrial Revolution would not have happened in Britain were it not for our natural supply of fast-flowing water. It fed our mills, became the steam that turned our engines and was channelled into thousands of miles of canals to transport our goods.
But tidal power, which has the ad-vantage over wind power of being predictable, has proved difficult to harness. However, an underwater windmill with sails 15 metres in diameter begins trials next year off Devon. If successful it could prove an important source of renewable power.
The country with the most awesome display of water power in the world is Iceland. There, water really shows off, gushing boiling from the ground, lying frozen in huge creaking glaciers or rushing in torrents over cliff-like waterfalls. The country's hydroelectric and geothermal power plants produce so much cheap power that bauxite is shipped halfway round the world from Australia to be smelted into aluminium.
Although supermodels regard it as a tonic for their complexion, drinking plenty of water is more than just a fashion thing - it's essential. Our bodies are three-quarters water, and if we become even slightly dehydrated we begin to feel faint and nauseous. If our body water content falls below 55 per cent we die.
Drinking any kind of clean water can help to keep our bodies in equilibrium and tap water does the job at a fraction of the price of the bottled variety. In the Drinking Water Inspectorate's recent annual report, 99.75 per cent of household supplies passed its stringent tests.
But perhaps the huge popularity of mineral water has more to do with a search for purity. The untainted appeal of fresh spring water has always been a selling point. An early recommendation for one of England's oldest brands claimed: "Malvern Water is famed for containing nothing at all."
But that's not strictly true. The term "mineral water" originated in the 1600s, when manufacturers began selling aerated water which they impregnated with salts in imitation of the spa waters popularly taken by the upper classes of the time. Some of these springs did actually contain beneficial minerals. Epsom salts remains the popular name for magnesium sulphate, even though the Surrey town from whose spring it was extracted in the 17th century has long since ceased production. Tonic, soda and barley water - still referred to as "minerals" in pub tariffs - are modern-day remnants of the 19th-century trend for adding flavouring to water to make it more palatable.
But today we like our water unadulterated, and sales of bottled water have recently overtaken those of fruit juice for the first time. And the curative properties of water are once again in demand as people are rediscovering old-fashioned luxuries such as steam baths and saunas, or experimenting with new-age treatments like colonic irrigation and flotation tanks.
After centuries as a symbol of rebirth and cleansing in religious ceremonies, water is acquiring a new spiritual dimension. Its spirits are worshipped by pagans and invoked by astrologers. The wrong kind of water in the wrong place can completely mess up your feng shui. Some people - hydrophobics - are scared of it while others take to it like a duck. Our heroes even walk on it. We take it for granted, but our lives depend on it. So let's all raise a glass - to water.