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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.

水能夠從一種形態變為另一種形態。太陽的熱從江河湖海中蒸發水，太陽的熱也從地表和植物、動物那里把水分蒸發出來，這樣就形成了水蒸氣，蒸汽上升形成了云。

當云接近高山時，被迫上升。隨著云的上升，云被冷卻。上升得越高，就變得越冷。這就使越來越多的水蒸氣凝結。如此，云中的水珠變得越來越大直到水珠變重，落到地面變成雨水。

落在地面上的一些雨水再次被蒸發，一些雨水滲入地下，這些雨水會被干早的作物吸收。也許會進入到井中或泉水中。大量的水又重新流人河流和海洋中。這一過程又重新開始。這個過程稱為水的循環。

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