From a cat's purr to a thunderstorm's clap, from a friend's voice to a school bell's clang, sounds can lull us, entice us or call us to action. But how does sound happen? How do we hear it? What makes some sounds loud and some soft? Some high pitched and some low pitched? How do humans and animals use sound to communicate? Which sounds happen naturally, and which are created for a specific purpose? This book explores all of these questions.