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.
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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.
Everything from the fibres in the Shroud of Turin to Otzi the Iceman has had their birthday determined the carbon-14 way. There's plenty of hydrogen, oxygen and nitrogen in living things too, but carbon's got something none of them do — a radioactive isotope that can take thousands of years to decay.
(You can read up on radioactivity and isotopes here).
Carbon-14, the radioactive version of carbon, is rare — it only makes up one trillionth of all the carbon in the world.
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.