Luis Kame, age 8, of Hoyden, Arizona, for his question:

What makes an echo?

You rear an echo when you stand and shout in dust the right place. Hello, you shout in a long empty hallway. Hello comes the echoing answer. Hello, you shout at the . face of a steep cliff. Hello, comas the echoing answer. No, there is not a pixie hiding at the end of the hall. There is not an elf in the cliff waiting to answer your friendly greeting.

An echo is a puzzling thing because you cannot see it happen. A bouncing ball is easier to understand because you can see it hit the floor and spring up again into the air. But the echo is something like the bouncing ball   except that you cannot see what goes on. This is because the clever trick is done by midgets far, far too small for your bright eyes to see.

Those midgets are molecules of air. Millions of these tiny particles of floating gas could sit on the head of a pin. In the air they float around like tiny balloons with plenty of space between them. In fact, these gas molecules are the air around us. We run  right through the air and, most of the time we forget it is there.

Tiny molecules are never still. They race and chase at a great rate crashing into each other. many times in a second, The slightest breeze, the smallest movement sends them helter skelter. You move billions of them every time you breathe. When you speak, you send as whole lot of  them bashing into their neighbors. The neighbors bang their neighbors and this crashing of air molecules is what makes the sound of  your voice moves out from your mouth in all directions.

Line after line after line of molecules crash into their neighbors in a mayor traffic dam.  Your voice crashes across the room. But if this molecule traffic jam tumbles down a hallway and meet a solid wall there is a bounce. The hard wall sets the sound moving back from where it came. It cones back to your ears as an echo.

Sometimes you can get a good echo from a steep cliff. Take 100 giant steps away from the foot of the cliff,, turn around and shout Hello. The sound of your voice travels to the cliff, bounces off the solid wall and comes back to your ears in about one second. The echo will be a little fainter than your voice because the sound gets weaker as it goes.

Sound travels through ordinary air about one mile in five seconds. It travels faster through water and still faster through steel. An echo travels at the same speeds. If your voice echoes back in five seconds, the sound has traveled half a mile to that steep cliff and half a mile back. Scientists use echoing instruments to test the depth of the ocean. The sound goes down through the water, bounces off the floor and comer back in so many seconds. The number of seconds tells how far it has traveled.