Water is common-place and precious. We take it for granted: it falls from the sky and is available from every tap in the land. It has a huge impact on life, and it lies at the heart of environmental changes engulfing our planet. In other words, water is a brilliant teaching resource - cheap, ubiquitous and providing a lead into many areas of the curriculum. As a bonus, because it feels interesting and has a tendency to slop around unpredictably, it has an inherent attraction for young children.
Water is a liquid compound of two gases - two parts hydrogen to one part oxygen, H2O. It has no colour or odour and takes the shape of its container. It is so basic that it was used to establish an international unit of measurement: one kilogram is the weight of one litre of water.
Billions of years ago, life is thought to have arisen in water. Astronomers look for its chemical signs to assess whether anything might be alive elsewhere in space.
All living things contain water. A human adult comprises about 65 per cent water - 40 or 50 litres, enough for a shallow bath. Our brains are up to 85 per cent water. Humans can survive for several weeks without food, but we would die within days without a drink.
Water under control
Water is the ultimate renewable resource, having been recycling itself for millions of years. Generations of children have drawn pictures of the water cycle, in which moisture on the Earth's surface rises into the atmosphere, cools, forms clouds, and precipitates back to earth as rain, when the whole process starts again.
It then flows into rivers (and eventually to the sea), or seeps down through the ground until it reaches the water table, below which the earth is permanently saturated. It can then be extracted by means of wells or boreholes.
All of this has been understood for centuries, and is constantly rediscovered by children digging holes on the beach near the tideline. What is new is our understanding of the extent to which human activities have affected these natural processes. Since the early days of human settlement, people have attempted to control their water supplies, often using elaborate irrigation systems for their fields. The Romans constructed aqueducts and systems of lead pipes to bring water into towns for cooking and bathing. The more complicated human societies became, the greater was their demand for water - not just to drink and for their crops, but for industry, sanitation and pleasure.
Today, we build dams, reservoirs and desalination plants. Water treatment plants and distribution networks provide nearly every home in the developed world with safe drinking water on tap. As a result of irrigation, there are racecourses in the Arabian desert and vegetable farms on the arid plains of California and Israel. We have swimming pools and fire hydrants, municipal fountains and domestic jacuzzis. It takes a litre of water to make a chocolate bar, nine litres to make the paper for a magazine like this one, and 100,000 litres to make a ton of steel.
In the UK, we use 18,000 million litres of water every day, but only 3 per cent of this is for drinking and cooking. We use 9.5 litres each time we flush the toilet, 80 litres to run the washing machine, and five litres per minute to have a shower. At more than 2,000 water treatment works around the country "raw" water is filtered, purified and disinfected before being piped into our homes through 400,000km of water mains. Two-thirds of what we use comes from surface water - reservoirs and rivers - and one third from underground sources reached by boreholes.
Water in dispute
A glass of tap water costs, on average, 0.03p in the UK. But in many countries, it is an unattainable luxury. A quarter of the world's population (1.4 billion people) do not have access to safe water. Diseases caused by unsafe water (diarrhoea, cholera, typhoid and parasitic infections) kill five million people every year in the developing world. Children are particularly affected. Not only are they vulnerable to diseases caused by dirty water (including the dangers to babies of drinking formula milk made with unsafe water), but fetching water is often the job of children, especially girls. Girls as young as 10 may have to walk three or four miles to the nearest supply, then carry home a container weighing several kilograms. The average person in the UK uses 135 litres of water a day, but in the third world it is just 10 litres.
Other problems arise where different countries share a water source. It is no coincidence that the word "rivals" comes from the Latin word for people who live on opposite banks of a river. If the people upstream pollute the water or take too much, the people downstream suffer. This is a simmering problem in several parts of the world. For example, the Blue Nile flows from Ethiopia through Sudan to Egypt. Ethiopia is the poorest of these countries, and could benefit from controlling the water supply; downstream, Egypt, which is a much more powerful country, views that prospect with alarm and has succeeded in discouraging international investment in Ethiopian water-management projects. In the future, nations could go to war over water supplies.
Water out of control
The most significant environmental issue facing the world today is climate change, and one of its momentous effects is to alter the distribution of water. Some places will get wetter, some drier. The impact on all living things is incalculable. The International Panel on Climatory Control estimates that sea levels will rise by up to a metre during this century. This will threaten one third of the world's crop-growing areas and the homes of one billion people.
Salt water will contaminate underground supplies of fresh water. Low-lying countries such as The Netherlands and Bangladesh, and islands such as Cyprus and the Bahamas, will be severely affected. In the UK, 57 per cent of productive agricultural land lies below the 5m contour line. Millions of homes will be subject to flooding. Parts of East Anglia, Teesside and the south-west of England could disappear permanently under the sea.
TES Primary's WorldWatch poster series illustrates some of the animals and habitats threatened by climate change. Water features in all of them. Even a slight change in its quantity or quality can make a habitat uninhabitable to the creatures that have adapted to it.
Key stage 1 Solid water
Ice is easy to provide. You can bring cubes into school in a vacuum flask; a little fizzy mineral water will discourage them from sticking together.
Try these activities:
* What happens as the cubes melt? Which go first - corners, sides?
* What does ink or water-based paint do to the ice?
* Does ice float? How deep? What changes as it floats?
* What part of the cube takes longest to melt? Who can keep their cube longest?
* How can you make the cubes stick together?
* You can make huge "ice balloons" by filling rubber balloons with water, sealing them, and putting them in a carrier bag (for safety) in the freezer. Peel off the balloon after the water has frozen.
Key stage 2: How far away is clean water?
In Kenya , it is not considered to be a man's job to carry water. A woman may have to carry 25kg of water for two or three kilometres, taking up to three hours a day and using one-third of the energy her daily food provides. As more villages are provided with pumps and piped water, life is changing for many Kenyan women.
lIf you have to walk 2.5km to get your water, you are likely to use around five litres a day.
lIf you have to walk 250m to get your water, you are likely to use around 25 litres a day.
lIf you have a water tap, you are likely to use around 75 litres a day.
Low water day
Hold a "low water day" using a minimum amount of water. Introduce a system of water tokens, for the pupils have to pay for the water they use.
New laws in England and Wales are intended to cut water consumption and improve its quality. The aims are: lto cut by 20 per cent the amount of water flushed by new toilet cisterns; lto cut the amount of water used in washing machines and dishwashers; linstallation of fewer products with high water use, eg power showers.
Surfers Against Sewage
Contact: Vicky Garner 01872 553001, e-mail: vicky@sas. org.uk; or visit their website: www.sas.org.uk. Vicky can put you in contact with a regionallocal SAS campaign.
Mersey Basin Trust
Contact: Val Cooper, education officer 0161 228 6924, or visit their website: www.merseybasin.org.uk. The trust is involved in activities to protect Britain's riverbeds.
Planet Pledge: www.planetpledge. co.uk. To help your local environment visit this site.
Water Watch www.waterwatch.org.uk.
For the latest news on everything related to water in the UK.
Wateraid This charity produces a pack for schools: tel 020 7793 4500.
The Friends of the Earth For detailed information on climate change: www.foe.co.uk.