To vibrate means to move back and forth very quickly, so quickly that the movement creates a blur or a hum. When your phone vibrates in your pocket, tiny motors inside make it shake rapidly. When a guitar string vibrates after you pluck it, it moves so fast you can barely see the movement, but you can hear the sound it creates.

Almost all sound comes from vibration. A drum vibrates when you hit it. Your vocal cords vibrate when you speak or sing. Even a tuning fork, which musicians use to find the right pitch, works by vibrating at a precise speed. The faster something vibrates, the higher the pitch of the sound it makes.

You can feel vibrations too. Put your hand gently on your throat while humming: that buzzing sensation is your vocal cords vibrating. A washing machine vibrates during its spin cycle. An earthquake makes the ground vibrate, sometimes so violently that buildings shake.

Scientists measure vibrations in hertz, which counts how many times something moves back and forth in one second. Middle C on a piano vibrates about 262 times per second. That's far too fast for your eye to see, but your ear hears it as a musical note.