When you drop a stone into a pond, you create circular ripples in the water—waves. The water bobs up and down, and crests and troughs spread outward in a circular pattern. If you drop two stones at the same time, the ripples interfere with one another. You can see this more clearly if you dip two oscillating pistons into a tub of water. When a crest from one piston runs into a trough from the other, the two cancel out; if you look carefully at the pattern of ripples, you can see lines of still, wave-free water (Figure 45).
