The radioactive isotope carbon-14 is created in the upper atmosphere when cosmic-ray particles from outer space strike nitrogen atoms and transform them into radioactive carbon.
Some of the carbon-14 might find its way into living creatures.
An example of the ingenious technical work and hard-fought debates underlying the main story is the use of radioactive carbon-14 to assign dates to the distant past.
For other examples, see the essays on Temperatures from Fossil Shells and Arakawa's Computation Device.
Love-hungry teenagers and archaeologists agree: dating is hard.