Potassium argon dating and carbon 14 dating
They are, obviously, assuming the amount of carbon-14 in the atmosphere has always been constant, and its rate of decay has always been constant.
Extensive laboratory testing has shown that about half of the C-14 molecules will decay in 5730 years. After another 5730 years half of the remaining C-14 will decay leaving only ¼ of the original C-14. In theory it would never totally disappear, but after about 5 half lives the difference is not measurable with any degree of accuracy.Plants breathe CO2 and make it part of their tissue.Animals eat the plants and make it part of their tissues.The C-14 in the plant or animal will begin to decay back to normal nitrogen.The older an object is, the less carbon-14 it contains.