Updated: Jul 6, 2020
When men, animals, and plants are alive they absorb carbon from the atmosphere. In this process, there is some carbon-14 also that is radioactive. Such radioactive materials send out small particles at a fixed rate, that can be measured. After a particular period, the radioactivity is reduced by half.
When a living thing dies, it ceased to take carbon-14 and continue to lose it at a fixed rate. Physicists have found the half-life of carbon-14 is 5730 years. A tree that was cut down or an animal that died 5730 years ago, would produce half as many clicks on the Geiger counter as a tree cut down or animal that died, yesterday. Geiger counter can detect individual atomic particles. It is used to detect and measure radioactivity.
If a piece of wood or bone is found in an excavation, we can measure its age with the Geiger counter. It is based on the amount of carbon-14 left in the ancient sample that scientists can tell its age.