What goes up must come down.

Everybody knows that. But what is it that pulls everything from rocks to rockets toward the center of the earth? It's gravity. Nobody can say exactly what it is, but gravity is there, pulling on everything, all the time. With the help of an adventurous scientist and his fun-loving dog, you can read and find out about this mysterious force.

Franklyn M. Branley was the originator of the Let's-Read-and-Find-Out Science series and the author of close to 150 popular books about scientific topics for young readers of all ages. He was Astronomer Emeritus and former Chairman of the American Museum of Natural History-Hayden Planetarium.