carbon dating shows how old a bone is by measuring the amount of carbon 12, and carbon 14 that the bone contains. and because carbon 14 is not replaced by exchange with the atmosphere the ratio of c12:c14 (the ratio of carbon 12 to carbon 14) tells how old the bone is by comparing the amount of carbon 14 to the amount of carbon 12. carbon 14 has a half life of 5,700 years, while carbon 12 is stable and does not radioactively decay at all.
