Radiocarbon dating is used to work out the age of things that died up to 50,000 years ago. As far as working out the age of long-dead things goes, carbon has got a few things going for it. The proteins, carbohydrates and fats that make up much of our tissues are all based on carbon.
Not only that, we top up our carbon-14 levels every time we eat.
And plants top up their radioactive carbon every time they turn carbon dioxide to food during photosynthesis.
It's not that the radioactive carbon in air or food doesn't decay, it does.
But something else is going on that keeps producing new carbon-14 — otherwise it would have all turned to nitrogen millions of years ago.
And that something else starts where Earth meets space.
Earth's upper atmosphere is constantly being bombarded by cosmic rays (usually protons travelling at nearly the speed of light).
When those speedy protons hit atoms you end up with a few stray neutrons zipping around the place.
Chemically, carbon-14 is no different from non-radioactive carbon atoms, so it ends up in all the usual carbon places — one trillionth of the carbon atoms in air, plants, animals and us are radioactive.
All radioactive atoms eventually decay into something more stable, and carbon-14 decays into nitrogen.
For a rare event it happens pretty damn often — one million carbon-14 atoms in your body decay into nitrogen every minute!
But don't panic — of the 800,000,000,000,000,000,000,000,000 carbon atoms in every one of us, about 800,000,000,000,000 are carbon-14, so we've got a few to spare.