He first noted that the cells of all living things contain atoms taken in from the organism's environment, including carbon; all organic compounds contain carbon.Most carbon consists of the isotopes carbon 12 and carbon 13, which are very stable.Atoms of the same element that have different numbers of neutrons are called isotopes. Most carbon on Earth exists as the very stable isotope carbon-12, with a very small amount as carbon-13.Here’s an example using the simplest atom, hydrogen. Carbon-14 is an unstable isotope of carbon that will eventually decay at a known rate to become carbon-12.
Carefully studying the Biblical record, we find that the universe has an age of around 6,000 years, and that a world-changing, global Flood occurred about 4,300 years ago.
Plants and animals assimilate carbon 14 from carbon dioxide throughout their lifetimes.
When they die, they stop exchanging carbon with the biosphere and their carbon 14 content then starts to decrease at a rate determined by the law of radioactive decay.
C in the atmosphere haven’t been constant throughout history (for instance, Noah’s Flood lowered the total amount of available carbon by burying lots of animals and plants).
So something that lived (and died) when the proportion of C was less than normal would appear to have died more years ago than it actually did (for example, it might give an age of 3,000 years before the present, rather than its true age of 2,000 years).