What is the Half life of carbon 14?
This is a remarkable result because it seems to imply thar carbon-14 should long since have disappeared from our billions-years-old planet. However, cosmic rays constantly react with nitrogen in the upper atmosphere to regenerate carbon-14, which then circulates down to us.
Organisms continually replenish their carbon-14 supply by drawing it in along with other carbon isotopes from the atmosphere or from food, but they stop doing so when they die. That is how we can judge the age of organic material from its carbon-14 content.