All organic material must stock up carbon atoms for various reasons, some of which randomly happen to be carbon-14. Scientists can use its level of radioactivity and compare it to the half life of carbon-14, which is the time taken for it to become half as radioactive as before. For Carbon-14, this is 5,730 years.

Why is carbon-14 is commonly used to help date organic material?

Because carbon-14 decays at this constant rate, an estimate of the date at which an organism died can be made by measuring the amount of its residual radiocarbon.