This method is known as carbon-14 dating and is based on the radioactive decay of this isotope.
Carbon is vital to all life on earth.
All living systems on earth utilize large amounts of the element carbon in their life processes. Carbon is the element which primarily makes up coal, charcoal, pencil leads, lamp-black, and many other familiar substances. When food is burned in a pot, the hard, black material remaining is primarily the carbon which the food contained. There are three isotopes or forms of carbon which occur naturally on earth. If sometime in the past, a jar containing one pound of carbon-14 had been placed on a shelf and had not been bothered since then, knowing that the jar originally contained one pound, we could measure the amount remaining in the jar and determine fairly accurately how long it had been there.