Tritium is a radioactive form of Hydrogen. Rather than bore you with the science, I'll gloss over it in a way that would make both your chemistry teacher and physics teacher blanch with disgust. However, your biology teacher will put a pencil in her mouth and look on longingly over her glasses because she's a dirty minx!
Sorry, I got sidetracked there.
Tritium replaced the bone-meltingly lethal Radium as paint on watch dials and is the T in T SWISS MADE T, the circled T on Military dials and the T in T < 25. It is used in two types of modern watch dials, with paint and with tubes.
Oh, and it's a WMD. No, I'm not joking, it's the kind of thing countries get invaded for not having. While it's replaced in more modern thermonuclear devices with Lithium Deuteride, it can still be used by developing nations to make sure their neighbours are less developing than them. Still, the radiation it gives off is rather gutless and limp-wristed. The crystal of a watch, even an acrylic crystal, blocks the harmful rads, meaning you'll not be gaining superpowers unless you eat your watch dials raw.
Anyway, back to watches. Being scarily atomic means it has a half-life, in this case one of 12 years - as in it decays to half its radioactivity every 12 years. Yes, this means a 1971 Rolex has 8 times less lume than it started out with, making a Superlume relume rather inaccurate looking.
Tubes, you say? Ah yes, Tritium gas is put in tiny glass tubes lined with fluorescent paint. These tubes are glued onto hands and dials and glow, whether you want them to or not, throughout their life, until they decay to nothingness. No charging needed, open the watchbox and there it is, glowing away.
So, to further the lesson and wrap it up so I can see what the Biology teacher has on her lesson-plan today, here are two pictures from my collection, the first of a watch with tritium tubes, the second with tritium paint.
Drop, 'em, blossom!
For further reading, try http://en.wikipedia.org/wiki/Tritium and http://en.wikipedia.org/wiki/Self-powered_lighting