system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What causes ripples, after throwing an object in water, to expand? What determines when they will lose enough energy to dissipate?
gpt
When the object enters the water, it pushes the water out of the way. It pushes as much water out of the way as the object is big. The only place for the water to go when it gets pushed is up, which is the splash that we see when the object enters.

Gravity pulls all that water back down, but it can't go down because there is more water in the way below it, so it starts to go sideways. This is the ripple you see. The water ripple spreads out in all directions across the surface.

As these ripples spread further and further away from the splash they take up more and more area of the surface of the pool. Because only as much water as the object was big was pushed up to make the ripple, the ripple gets shorter and shorter as it spreads out over more area. When the water is all together it makes a big pile, but the pile gets shorter and shorter as you try to spread that same amount of water over bigger and bigger areas.

Eventually, the water spreads out over such a large area that you can't even see the ripple any more. As the ripple moves around it also has to push against air that's in the way, but this is a lot easier than pushing the water out of the way so you don't see it as much. But the more air the ripple pushes out of the way, the slower it moves. After a long time, the air slows the ripple down so much that it stops and the pool surface is flat again.