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.

水能够从一种形态变为另一种形态。太阳的热从江河湖海中蒸发水，太阳的热也从地表和植物、动物那里把水分蒸发出来，这样就形成了水蒸气，蒸汽上升形成了云。

当云接近高山时，被迫上升。随着云的上升，云被冷却。上升得越高，就变得越冷。这就使越来越多的水蒸气凝结。如此，云中的水珠变得越来越大直到水珠变重，落到地面变成雨水。

落在地面上的一些雨水再次被蒸发，一些雨水渗入地下，这些雨水会被干早的作物吸收。也许会进入到井中或泉水中。大量的水又重新流人河流和海洋中。这一过程又重新开始。这个过程称为水的循环。

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