Water can change from one form to anotheer. The heat of the sun evaporates water from the oceans, seas, rivers and lakes. The heat also evaporates water from the ground and from plants and animals. Water vapour is formed. This forms clouds in the air.
When the clouds come near a mountain, they are forced to rise. As the clouds rise, they are cooled. The higher they rise, the more they are cooled. This causes more and more water vapour to condense. In this way, the tiny drops of water in the clouds get bigger and bigger until they are heavy enough to fall to the earth as rain.
Some rain water which falls on the earth is again evaporated. Some of it sinks into the ground. It may be used up by thirsty plants. It may reach a well or a spring. Most of the water goes back to the rivers, seas and oceans. This process then starts all over again. This process is called the water cycle.