One of the characteristics of the planet Earth which makes it unique is the fact that it has liquid water on its surface. Most of the water in the Earth's hydrosphere is found in the oceans (more than 97%). About 2% of the water is frozen as ice and snow, mostly near the North and South Poles. The rest of the water, less than 1 %, is underground, in lakes and reservoirs, rivers and streams, marshes and swamps' and inside the living organisms that make up the biosphere.

Water can be found everywhere in the environment: in the atmosphere, in soil, in plants and animals, in the human body. Water is essential to daily living. The water which comes out of the kitchen tap seems never to run out. Where does all that water come from?

The planet Earth may be considered as a closed system where the water within it is continuously recycled. No water enters this recycled on Earth for millions of system from space and no water is lost from it yea's. to space. The water people drink today is millions of years old and it will continue to be recycled for millions of years. This cycle is called the water cycle.

1. Water evaporates from huge reservoirs known as the seas and oceans. This water evaporates because of the heat from the sun. The water which is left behind is `salty' because all the dissolved substances in the water, called mineral salts, are left behind. This means that the water that is evaporated is purified water.

2. Looking out over the sea, it is impossible to see drops of water moving upwards into the sky. This is because the water which is being evaporated is turned from a liquid form into another form: a gaseous form called water vapor. It is this water vapor that makes the air humid. Sometimes it is possible to feel that the air is humid, but it is impossible to see that water is carried in the air. Water may, of course evaporate in the same way from water sources other than the oceans such as lakes or rivers.

3. As the air carrying the water vapor moves higher and higher, the air becomes cooler and cooler. When the temperature of the air drops to a certain temperature, the water vapor starts to condense into liquid water again in the form of little droplets. When there is enough condensation, it is possible to see the water droplets because they come together as clouds. The more evaporation there is, the more water vapor condenses and so the clouds become bigger and bigger.