Nearly 700 different muscles control the daily life of a human body. And believe it or not, whether a muscle is big or small, it's made up of just one cell. Exercise doesn't give people more muscles, but it strengthens the ones people have. Veteran nonfiction writer Seymour Simon takes young readers on a tour of the human body. They'll discover how muscles make it possible to move -- and see what's really going on underneath the skin.