How are radioactive isotopes used to date rocks?

Some isotopes, however, decay slowly, and several of these are used as geologic clocks. Dating rocks by these radioactive timekeepers is simple in theory, but the laboratory procedures are complex. All methods rely on the fact that certain elements (particularly uranium and potassium) contain a number of different isotopes whose half-life is exactly known and therefore the relative concentrations of these isotopes within a rock or mineral can measure the age.

The answer is constant.