People tend to associate “what if” questions with late-night arguments and science fiction, but they can be a real part of science, too.  They are called “counterfactual” questions or situations.

As an example, let’s ask “Why is the top of a pond flat?”  The easiest way to answer that question is to visualize a pond that isn’t flat on top, perhaps one with a little hill of water.  What happens?  The water in the hill runs downhill, of course, so the hill won’t stay there.  In other words, you assume a counterfactual hilly shape, and it is immediately obvious that the water won’t stay there, so you picked the wrong shape.  None of these hilly shapes are stable.

The only shape for the water’s surface that doesn’t have that problem is a flat surface.    So, flat surfaces stay flat; all other surfaces change.  That means you end up with a flat surface if you wait long enough for all the waves to damp themselves out.