Tritium Decay rates

Tritium is just hydrogen. You may remember from school that an atom has a nucleus (where the heavy positive protons and neutral neutrons live) and a number of negative electrons orbiting it. An atom's chemical behavior comes from the number of electrons it has, which is by default the same as the number protons in the nucleus (balancing the positive and negative charges).

Your basic generic hydrogen atom has just a proton and an electron, no neutrons. But the nucleus can accommodate one or two neutrons alongside the proton... for a while, anyway. The resulting atoms are heavier but otherwise chemically the same as basic hydrogen (they each have a single electron). With one neutron is called "deuterium", with two, is called "tritium" (because the atoms are roughly twice as heavy and three times as heavy as entry-model hydrogen, respectively).

So one or two neutrons can barge into the proton's one-bedroom nucleus and crash on its couch. And with just one extra, it's stable: deuterium will happily remain that way indefinitely (and about one hydrogen atom in 6420 is deuterium: your body is about 1/10 hydrogen, so if you weigh 50 kilos, it's about 16 grams deuterium). But there is a crowd, so two neutrons on the couch is too much, and sooner or later they have to go: it's entirely random when it happens, but if you have a bunch of tritium atoms and put them in a storage locker, when you come back to clear it out 12.3 years later, half of them will have decayed (they turn into helium, which sort of breaks the couch-surfing metaphor, but there you go).

Tritium is useful in watches because that constant rate of decay can be harnessed, with a little phosphor, into a continuous glow which requires no external charging or power source. It does wear out, both because the tritium inherently gets used up by half every 12.3 years, but also because (if I understand correctly) the decay products degrade the chemical lume.

Tritium is not particularly dangerous, much less so than radium, which has also been used for watches. It's probably not a good idea to inhale it or drink water which has tritium in it. It occurs rarely in nature (sometimes when hydrogen in the atmosphere gets slammed with a cosmic ray particle, you get tritium). It's manufactured by using a nuclear reactor to irradiate lithium, breaking those happy nuclei into doomed tritium households. It's controlled because it's a component of thermonuclear weapon designs.