What is nuclear fission?

Fission is the splitting of a heavy nucleus — usually uranium-235 — into smaller pieces. A neutron strikes the nucleus, it splits, and out comes a burst of heat plus a few more neutrons that go on to split more atoms: a self-sustaining chain reaction. The tiny bit of mass that vanishes in the process becomes an enormous amount of usable energy.

It's a genuinely awesome piece of physics, and the fact that we can hold this reaction perfectly steady for years at a time is one of engineering's quiet triumphs.