Why is radioactive decay a good method of measuring the absolute age of ancient fossils?

There are different types of atoms with the same chemical properties but with different numbers of neutrons in the atomic nucleus. These are called "isotopes". The most common element in most living organisms is carbon, which comes in two isotopes; carbon 12, which is normal ordinary carbon and carbon 14, which is very slightly radioactive. The radioactivity means that it will decay, changing into some other element, very slowly.

The proportion of carbon 12 to carbon 14 is pretty steady, so as living things grow, they take in carbon 12 and carbon 14 in their food and air. When the plant or animal dies, they stop taking in new carbon. The carbon 12 remains the same, while the carbon 14 decays into something else very slowly. When we find the remains or the fossil of the dead animal or plant, we can measure how much carbon 12 and how much carbon 14 there is, and calculate how long it has been since the animal died.