Our transcription: The rate of decay of any radioisotope is measured in terms of its half life -- how long it takes for half of its atoms to decay.
The half life of carbon-14 is 5,730 years.
Carbon-14 is most commonly preserved in the form of charred wood that is buried by sediment, lava, or volcanic ash.
If a sample contains only half the level of carbon-14 found in living organisms, geologists conclude that it must be 5,730 years old.
If the ratio of carbon-14 is only a quarter of that of a living organism, another half life has elapsed, and it must be 11,460 years old, and so on.
This method is only reliable for dating things back about 50,000 years.
Beyond that, the amount of radiocarbon remaining is too small to permit accurate measurement.