Hills are waves
From the book, The Web of Meaning by Jeremy Lent:
Think of a landscape with undulating hills, going off into the distance. Now turn your mind to the swell of waves on the seashore. Because of the speed with which the waves move, you can easily recognize their momentary form as a function of their temporal pattern, but the hills that seem so fixed are essentially land-based waves, just moving a million times more slowly than the ocean waves, so we see them as static.