system
Given the following post, respond as if you're commenting on Reddit:
human
How does carbon dating accurately work considering all of the molecules likely come from beyond ancient supernova?

I'm just confused.
gpt
Don't worry about supernova and the formation of the earth. Carbon dating is waaaay simpler than that. 

High energy cosmic rays are constantly bombarding the earth from space which convert a little bit of the Nitrogen-14 in the atmosphere into Carbon-14. That carbon-14 is radioactive and decays away slowly, with a half-life of about 5700 years. 

What this means is that there is an approximately constant fraction of the carbon in the world which is this radioactive carbon. It gets taken from the air and used in photosynthesis by plants, and so all living things should have this same fraction of 14C in them as long as they are alive and actively exchanging matter with the environment. When something dies its 14C begins to decay away and by measuring how much 14C is left in some dead thing (either by counting the decay rate with a Geiger counter or running it through a mass-spectrometer) we can measure how long ago it died.