When living things (animals & plants) are alive they havethe radioactive chemical carbon 14 in their bodies. Radioactivematerials emit radiation. When it dies the carbon 14 ceases to berenewed and slowly begins to emit less and less radiation. That is,the radiation decays and it decays at a known rate. So by measuringthe amount of radiation being emitted by something long deadscientists can tell how long ago it died.