Why is carbon-14 found in all living things?

Answer 1

Carbon-14 is produced by the effect of solar radiation on nitrogen-14 in the atmosphere. This is then incorporated into plants during the process of photosynthesis. Animals eat plants (and/or other animals that have eaten plants), so carbon-14 gets into their bodies as well.

Answer 2

A living being has a constant amount of C-14 throughout its lifetime as the decaying C-14 is constantly being replenished either through photosynthesis for plants or food consumption for animals or through breathing ensuring that all living things have approximately the same percentage of c-14 in their bodies.

Answer 3

Carbon-14 is only one form of carbon; others have differing mass numbers, e.g. carbon-12, carbon-15. Thus it fits the definition of an isotope.