Put a thin layer of water in a 9x9 baking pan, add some food coloring, and slosh it back and forth slowly. This is a illustration of wave motion.
The computation below shows the sort of pattern that arises if you slosh the pan gently in the diagonal direction, at one instant in time.
Sloshing slightly faster gives a more interesting wave pattern. Here's what it might look like from above, with darker blue corresponding to deeper water.
Computed using CLAWPACK