In the heat of the sun, water evaporates from seas, rivers and lakes. As water vapour rises, clouds form in the air. When the clouds rise high, they get cool. Finally, they become drops of water. When they are heavy enough, they fall to the earth as rain.
在太阳的照射下，水从大海、河流和湖泊上蒸发。当水蒸气上升时，云就在空中形成了。当云升高后，就变凉了，最后变成了一滴滴的水。当水变得足够重时，就作为雨水落到地面上。

Some rainwater sinks into the ground. It's good for the thirsty plants. Most of the water goes back to the seas, rivers and lakes, and the water cycle will start all over again.
有些雨水深入到地面，这对干渴的植物有益。大部分水又返回大海、河流和湖泊。然后水的循环过程将再进行一次。

词组解析：

1.cycle：循环
2.heat：高温
3.evaporate：使蒸发
4.vapour：蒸汽
5.rise：上升
6.drop：滴
7.heavy：重的
8.fall：落下
9.rainwater：雨水
10.sink：渗透
11.thirsty：口渴的
12.again：再一次