Sherry Kowal, age 10, of Winnipeg, Manitoba, Canada, for her question:

What exactly are sound waves?

Years ago, people had a sort of riddle about sounds. When a tree falls in a forest where nobody is near to hear, does it make a sound? Most said it could not make a sound unless there were ears to hear it. Nowadays, we know that old tree certainly came down with a noisy crash.

To understand sounds, we must imagine a crowd of things that are much too small for our eyes to see. These midgets are mini particles of matter called molecules. We also must imagine a peppy something called energy    which is quite invisible. Even though we cannot see them, molecules and energy work together to create various sound waves that whisper, shout and mumble in our ears.

All sounds must travel through something made of crowded molecules. The something may be either liquid, like water, or solid, like steel or gaseous, like air. Sounds start out as spurts of energy and the mini¬molecules are needed to carry it along. Suppose, for example, you bang on a drum. Your peppy energy causes the drum head to vibrate or tremble.

These tiny vibrations bash the air, causing its molecules to vibrate also. The jogging molecules jog their neighbors    and the sound energy is carried farther and still farther from the drum. It travels out and away as the jogging molecules push and relax, push and relax. These bouncy jogs are called sound waves.

Different sounds bounce along in different waves    long or short, fast or slow. Fast waves that pulse many times per second bring high notes to our ears. Slower waves, with fewer pulses per second, bring us the lower notes.

How fast the molecules carry sounds depends on how close they are crowded together. For example, sounds travel faster through water than through air, faster through steel than through water.

In air, through which sounds travel quite slowly, the molecules are widely separated from each other. In water, which carries sounds faster, the molecules are more crowded. The molecules in steel are jam packed together and they can bounce the pulsing sound waves along very fast indeed.

The speed of pulsing sound waves is called frequency. It measures the number of times the sound energy pulses in a second. The lowest note on a piano has a frequency of 27, which means its sound waves pulse 27 times in a second. The highest notes we can hear have frequencies of 20,000    pulses per second.

These are just a few simple examples of how sound waves behave. Actually they can do many more complicated things. For example, the amount of energy in a sound makes a difference. If you bang a drum with all your might, it sounds very loud. But it sounds much softer to somebody down the street. This is because sound waves gradually lose their pep as they travel farther from where they started. Finally they fade away.