What if--

Discusses what might happen if an asteroid hit the Earth.

An imaginative look at space with such questions as "What if the sun went out?" and "What if there were Martians?".

Explores reasons why humans might want to live in space, as well as technical problems and scientific considerations involved in establishing a space colony.

Text and accompanying illustrations answer basic questions about the parts of the body and how they work.

Through questions and answers, takes a look at Earth from its very beginnings to what it is like today.