Echo... Echo... Echo...

We went to Lake Martin with my roommate today.  As we were playing with the family's dog, he started barking and it echoed back to us.  Sound is created through vibrations in the air, and echo happens when these vibrations bounce off of a surface.  The vibrations caused by the dog barking traveled through the air, hit a surface, bounced off of it and traveled back to us. Which is why we the dog barked once, but we actually heard the sound three times!