Did you ever throw a rock into a pond and see the ripples in the water made by the rock? Those ripples are motion waves.

When you talk or shout your sound also travels in waves. These are sound waves but we can't see them.

When motion waves made by the rock reach the shore of the pond, the waves bounce back. 

Sound waves bounce back, too.  We call this an echo.  Did you ever shout in a long hallway and hear your voice echo?

Bats do that when they fly.  Bats fly with their mouths open.  They make noises and listen for the sound to bounce back - they listen for the echo.

Bats are very good at hearing echoes - much better than people!  When they hear the echo, they can tell where an object is, how big or little it is, and how far away it is.  They can tell if the object is moving.  They can tell what direction it is going.

This is called echolocation.  Echolocation is using sounds and their echoes to locate objects. 

If the object is a tree or a building, bats can keep from bumping into it.  If the object is a bug, they can find it and catch it for dinner!

Dolphins, whales, seals, porpoises,and some birds use echolocation, too.