A water mill is like a big toy that uses water to make things work. It looks like a big building with a big wheel on the side that spins around. You can imagine it like a bicycle wheel, but much bigger!
When water comes near the wheel (which is usually in a river), it makes the wheel spin around. The wheel is attached to a bunch of gears and other things inside the building, which makes them move too. This is called turning the mill.
The turning of the mill is really helpful for people because it can be used to do a lot of things. For example, in the olden times, people used water mills to grind up grain into flour to make bread. The spinning of the wheel would power the grinding stones inside the building, which would turn the grain into flour.
Nowadays, water mills are not used as much because we have other ways to do things that are easier and faster. But they are still really cool to look at and some places still use them for special things, like making special kinds of flour or other kinds of crafts.