David Breidenbach, age 12, of Bixby, Oklahoma, for his question:

What is the difference between fission and fusion?

Both terms, of course, refer to changes in the atomic nucleus. They differ because one means splitting and the other means joining. Just remind yourself that an earthquake can cause a ghastly split in the ground called a fissure. For a back up reminder, think of fusing two pieces of metal together. Naturally, these mundane reminders are far removed from nuclear activity. But there is a relationship.

In nuclear fission, the atomic nucleus is split and perhaps shattered. Neutrons and other atomic fragments become high speed bullets. Some strike and split the nuclei of other atoms, creating a chain reaction of nuclear fission. The shattering process releases dynamic nuclear energy. The best example of nuclear fusion goes on in the sun. There the atoms of hydrogen fuse together to form larger atoms of helium. A portion of the hydrogen material is not needed by the remodeled helium atoms. In the fusion process, this leftover portion of matter is converted into nuclear energy