Rick Ishler, age 11, of Rock Island, Calif., for his question:

WHY DO WE HEAR AN ECHO?

An echo is a sound that we hear after it is reflected or bounced back from some object. We first hear the sound when the waves reach our ears. Then, if the sound waves also hit some large object, such as the side of a building, they bounce back and may reach our ears  a second time. The second sound is called an echo.

Sometimes we do not hear echoes even though the reflected sound waves reach our ears. We may not hear an echo if the original sound is too weak or if the reflecting object is too small.

Sometimes we may not be able to tell the difference between the sound and its echo if the reflected object is less than about 30 feet away.

At other times, we may hear more than one echo from just one original sound. Such repeated echoing usually occurs in valleys and canyons where there are many sound reflecting surfaces. The sound waves bounce from wall to wall and then often produce several echoes.

It takes 10 seconds for sound to reach an object a mile away and return