Carbon-14 is also passed onto the animals that eat those plants.
Plants and animals naturally incorporate both the abundant C-12 isotope and the much rarer radiocarbon isotope into their tissues in about the same proportions as the two occur in the atmosphere during their lifetimes.
When a creature dies, it ceases to consume more radiocarbon while the C-14 already in its body continues to decay back into nitrogen.
C-14 is produced in the upper atmosphere when nitrogen-14 (N-14) is altered through the effects of cosmic radiation bombardment (a proton is displaced by a neutron effectively changing the nitrogen atom into a carbon isotope).
The new isotope is called "radiocarbon" because it is radioactive, though it is not dangerous.
Until this century, relative dating was the only technique for identifying the age of a truly ancient object.