system
Given the following post, respond as if you're commenting on Reddit:
human
Why does radioactive dating not date things to when they were created inside a star?

How can we tell when a sedimentary layer, fossil, etc was created, when that substance's nuclei have been decaying since it was created via fusion before the Earth came about? Does the substance get reset so to speak, when it is melted? I would intuitively guess not, as heating substances a few thousand degrees celsius wouldn't have an affect on the nuclei.
gpt
For carbon dating of biological samples, we use the fact that carbon-14, which is unstable, is continuously formed in the atmosphere due to interactions between cosmic rays and (iirc) nitrogen. Therefore, living things which are cycling carbon through their bodies will have a certain ratio of carbon-14 and carbon-12. Once these things die, they are no longer cycling their carbon, and the carbon-14 gradually decays and changes the ratio. By looking at the ratio of carbons, it is possible to figure out how long ago something stopped cycling its carbon.