Jerry McCrains age 10 of Asheville,, N.C., for his question:

Many a fast plane can travel a mile in five seconds. But at this speed, the filmy air acts like a solid wall. The plane must crash through this strange wall and it does so with a clap of thunder. The noise reaches our ears a few seconds later. Aha, we say, some high flying pilot has just broken the sound barrier. Some call the loud noise a sonic boom. We look up, but the plane Is far out of sight. Maybe the pilot is up there scouting to protect our land from surprise attack.

A plane meets the sound. barrier when it reaches the speed of sound. Up until that point, it slices through the air with no trouble at all. Actually, the barrier is not caused by sound at all. It is caused by the air. Air, like everything else, is made from tiny molecules, far too small for our eyes to see. Sound uses these molecules to travel from place to place. Because of them, it can travel just so fast and no faster.

The laws of sound are not easy to understand, for we cannot see it in action or touch it. We can only hear a sound if we happen to be around when it happens, If there are no ears to hear it, a great tree falls silently in the lonely forest. Sound also needs something through which to travel. We call this a medium. It can travel through air, through liquid water, through steel and other solids. But the face of the moon is silent. For there is no air or other medium there through which sounds can travel.

A beating drum, for example, starts up a sound in the air of a room. It jostles the tiny air molecules next to it and they jostle their neighbors. Their neighbors jostle their neighbors and the sound of the drum is carried out in all directions by the jostling molecules.

But these molecules can move so fast and no faster. Through ordinary air, they carry the sound waves at a speed of about one mile in five seconds. Through water, where the molecules are closer together, sound travels at about a. mile a second. Through steel, in which the molecules are packed very closely together, sound can travel three miles in a second.

The speed of sound, then, through the air, is about one mile in five seconds. The jostling molecules can move this fast, but no faster. The fastest runner takes about four minutes to run a mile. So as we move through the air, the little molecules can get out of the way with ease.

But what happens when a plane moves too fast for them? At five seconds a mile, the speed of sound, the air molecules cannot move fast enough to get out of the way. The speeding plane pushes them and they crowd together in a wall of dense air. At the speed of sound, the plane seems to reach an impossibly strong head wind. But the modern plane is streamlined, knife‑edged and powered with mighty engines. It uses its energy to slice through the barrier with a. clap of thunder.