(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.
So the proportion of carbon-14 inside living things is the same as the proportion of carbon-14 in the atmosphere at that time.
But when we stop eating, or when plants stop photosynthesising, our carbon-14 levels no longer get topped up.
And that something else starts where Earth meets space.