Christopher Gough, age 12, of Staten Island, New York, for his question:

Why is sound a form of energy?

We tend to forget two things that all forms of energy have in common. They are soundless and invisible. Oh, yes, they are. We can hear the bang of a pounding hammer    but not the energy that powers it. We can see a burning bulb, but not the electrical energy that lights it. So it's not surprising that the sounds we hear are caused by some silent, invisible form of energy. And this energy must have something to jog as it moves along, a medium through which to pass.

It is created when some other form of energy sets the molecules of a medium jogging with vibrations. This energy may come from arm muscles beating a drum. The drumhead vibrates, bashing the molecules of the air. The energy of these moving air molecules creates sound vibrations and relays them outward in widening spheres. As this sound energy spreads, it grows weaker and finally fades away into silence.