Process o radioactive dating
The carbon-14 it contained at the time of death decays over a long period of time.
Many rocks and organisms contain radioactive isotopes, such as U-235 and C-14.
These radioactive isotopes are unstable, decaying over time at a predictable rate.
As the isotopes decay, they give off particles from their nucleus and become a different isotope.
The half-life is the amount of time it takes for one half of the initial amount of the parent, radioactive isotope, to decay to the daughter isotope.
Thus, if we start out with 1 gram of the parent isotope, after the passage of 1 half-life there will be 0.5 gram of the parent isotope left.