Radioactive isotopes are used to measure fossils and other artifacts. While an organism is alive, it takes in isotopes. Once the organism dies, the isotopes do not enter the body anymore. Scientists can estimate how much of the radioactive isotopes were present in the body to begin with and then determine how much is currently left.
"Radiochemical dating techniques are the most common method for age dating sediment cores over years or decades."
by Korrok November 1, 2007