Q: Why does carbon dating detect when things were alive? How are the atoms in living things any different from the atoms in dead things?
Physicist: As far as carbon dating is concerned, the difference between living things and dead things is that living things eat and breathe and dead things are busy with other stuff, like sitting perfectly still. The vast majority of carbon is 12C (carbon-12) which has 6 protons and 6 neutrons (12=6+6). Chemically speaking, those 6 protons are far more important since they are what makes carbon act like carbon (and not oxygen or some other element). The extra pair of neutrons do two things: they make 14C heavier (by about 17%), and they make it mildly radioactive. Living things use carbon a lot (biochemistry is sometimes called “fun with carbon”) and this carbon enters the food chain through plants, which pull it from the air.