Hank O'Connor, age 14, of Flagstaff, Ariz., for his question:

WHAT IS FISSION?

Cellular fission is a form of reproduction. It happens in certain single celled plants and animals, such as protozoa and bacteria, and in a few many celled animals, such as those that produce coral.

In binary, or simple fission, the organism divides into two equal parts. In multiple fission, it divides into more than two parts.

Nuclear fission is the splitting of the nucleus of an atom into two parts. Fission can occur when an atomic particle, such as a neutron, strikes the nucleus of an atom. The atom may split into two smaller atoms. When this occurs, atomic particles and energy are released.

The fission of certain heavy atoms, such as uranium and plutonium, produces the energy for an atomic bomb.