Though people seldom witness it directly, the entire world is in a state of motion, and where solid objects are concerned, this motion is manifested as vibration. When the vibrations produced by one object come into alignment with those of another, this is called resonance. The power of resonance can be as gentle as an adult pushing a child on a swing, or as ferocious as the force that toppled what was once the world's third-longest suspension bridge. Resonance helps to explain all manner of familiar events, from the feedback produced by an electric guitar to the cooking of food in a microwave oven.