All living things -- animal or vegetable -- contain large amounts of carbon.
C-14 is formed when cosmic rays knock neutrons out of carbon atoms in the Earth's upper atmosphere.
After about 10 millennia, only one quarter of the C-14 atoms remain.
This means that a living plant or animal has only one Carbon 14 atom for every trillion Carbon 12 atoms.
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Some of these neutrons hit Nitrogen-14 nuclei, causing a proton to be ejected, and converting them to C-14.In a process called photosynthesis, plants use the energy in sunlight to convert carbon dioxide from the atmosphere into carbon-bearing compounds that the plant can utilize.