We usually learn about the Water Cycle in Years 3 and 4.  It explains the process of how the same water falls as rain and then returns to the clouds through evaporation.  This BBC Bitesize video explains the different stages of the Water Cycle.

By Year 6, there are some extra words to learn.  As well as the words we learnt in Years 3 and 4, this image also shows some more specific language:

• transpiration is water that evaporates from plants and trees.  So much of the planet is covered with vegetation that this is a significant source of evaporation.
• infiltration is when water drains into the ground rather than being directly evaporated off and can be stored underground for thousands of years as groundwater.
• surface runoff is when rain falls on a hard surface and is unable to infiltrate the ground.  It runs along the ground (with some being evaporated off) and ends in rhynes, streams and rivers, or ends up somewhere else where it may infiltrate.

This video explains how the same water molecules keep going round and round.  It is a Youtube video, so please only watch it with adult supervision.