So, we have a “clock” which starts ticking the moment something dies.
Obviously, this works only for things which were once living.
In the last video, we talked about the idea that if I dug up a bone someplace, if I dug up a bone, and if I were to measure its carbon-14, and I found that it had half of the carbon-14 that I would expect to find in a living animal or plant, that I said, hey, maybe one half life has gone by, or roughly for carbon-14, one half life is 5,730 years.
So I said maybe it's 5,730 years since this bone was part of a living animal, or it's roughly that old.
There are three carbon isotopes that occur as part of the Earth's natural processes; these are carbon-12, carbon-13 and carbon-14.