Carbon 14 is an isotope of carbon. It is present in the air, and is radioactive.
When we breathe, we take in tiny amounts of it into our body. In fact, every living thing is constantly exchanging Carbon 14 with its environment.
Once a being dies, this exchange stops. The car bon remains in the body until it decays, or becomes a fossil.
The Carbon 14 that is present in the decaying or fossilized organism gradually decreases, because of radioactive decay.
Scientists can therefore, estimate the age of a fossil by looking at the level of decay of the radioactive carbon it contains. This is known as carbon dating.