What are noises made of?

   It's easy enough to hear noises, but you can actually see how a noise is made if you have a toy drum. Put a few bread crumbs on the drum. Then hit it gently. You hear a soft noise. And you can see the crumbs dancing a little. Now hit the drum harder. You hear a louder sound. The crumbs bounce higher. The noise and the bouncing were made by the same thing. They were made by the top of the drum, which began to wiggle up and down very fast when you hit it. We say the drum vibrated. A gentle hit made it vibrate a little. A hard thump made it vibrate a lot.
   How did your ears know that the drum was vibrating? The answer is easy if you remember that there is air all around both you and the drum. When the drum top vibrated, it pushed up on the air and made it move. The drum started a lot of little moving waves in the air. We call them sound waves. And they traveled all the way from the drum to your ears.
   Each of your ears has a drum in it, too. An eardrum is a tiny piece of skin stretched tight across an opening inside your head. Of course, an eardrum can vibrate. When sound waves hit your eardrum they made it vibrate. Your vibrating eardrum tells you that the toy drum made a noise.