Billy Davis, age 7, of Louisville, Kentucky, for his question:

How does radar work?

Did you ever hear an echo? It may happen in a canyon with steep cliffs along the sides. In such a place, when you shout a loud "HELLO!" your voice comes back with a softer, "Hello." This is an echo. The sound of your voice spreads out and reaches a cliff. It could go no farther, so it bounces off the hard wall and comes back. Radar works in somewhat the same way, but it does not use the same sort of sound that we use for talking. Instead of these ordinary sound waves, radar uses the energy of radio waves.

A radar system has several complicated parts to do its work. It must be able to send out a radio beam and direct it to sweep across the sky. If there is nothing there but empty air, it travels on and on and gets lost. But if it strikes a solid plane, the radio beam bounces back. The radar system must be able to catch the returning echo. Special electronic gadgets can change it into a spot and show it on a screen. The spot that echoes back tells where a distant plane is    and lots of other things about it