It is the process by which water changes its state to replenish various water sources.
water cycle is a continuous cycle  were water evaporates,travels into air  and  becomes part of cloud,then falls down as precipitation ,and then evaporates again.this cycle is repeated again and again .water cycle helps in replenishment 
in water sources like lakes ,rivers etc.
i hope this helps u....
The water from the sea will be evaporated, and it will condense in the air, and then, fall back on the earth as rain. Then, the same process is repeated again and again.
This is known as the water cycle, and is repeated again and again.

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Hope this will help you.