When it rains water will flow from the top (The Headwater) and will flow down which is called a runoff.
The water will flow through streams, rivers and other water areas and will flow into a bigger body of water.
You may not know it but some of the water will soak into the ground. This water puts pressure on the ground and holds us up right now. This water can even be taken out by an aquifer and drank by you.
You all know pollution is bad but do you know how it can be spread into lakes, rivers or oceans. Well, I will tell you.
Pretend the red cube is garbage when it rains the water will pick up the garbage and carry it into the body of water.
How this can happen forever is water evaporates into a gas then the gas cools in the air and clumps into a cloud through condensation. When the cloud has enough water it will let out rain which is percipitation.