Carbon-14 age dating
Carbon-14 has a short half-life (~5730 years)
Carbon-14 is always being made in the earth's upper atmosphere by a neutron hitting a 14N atom with the neutron being added to the atom.
As a proton is kicked out forming carbon-14, the carbon-14 decays to nitrogen
The caron-14 combines with oxygen to form carbon dioxide, which is used by plants during photosynthesis.
As long as plants and animals are alive they will have a constant amount of carbon-14.
After death the amount of carbon-14 starts to decline
The carbon-14 system works well on dying bones, wood, charcoal and other organic materials that are younger than ~50,000 years.