This method can even be used on dead trees that fell in a forest long ago.
For example, the last 200 rings in the dead tree might match up with 200 rings early in the life of the living tree, so the two trees together can count back many years.
The half-life is the time that it takes for half the radioactive sample to change from one element into the other.
Some isotopes have short half-lives of minutes or years, but Potassium-40 has a half-life of 1.3 billion years.
As time passes, a rock will have more and more Argon-40 and less and less Potassium-40.