system
Given the following post, respond as if you're commenting on Reddit:
human
When we carbon date items, how do we know how much carbon started in them?
gpt
We aren't measuring the amount of carbon, we are measuring one isotope, carbon fourteen.  C-14 is formed by cosmic rays hitting the atmosphere.  Carbon fourteen is unstable, it has a half life about five thousand years.  Plants get their carbon from the atmosphere, animals get their carbon from plants, living things have an exact ratio of carbon 14 to normal carbon.  It doesn't change during their lifetime, because they continue to grow and take in new carbon.  But once they die, they stop taking new carbon 14 in, and the C-14 begins to decay.  A 5000 year old object has half the c14 of a fresh one; it is a quarter when it is ten thousand years old. By fifty thousand years, there is too little to measure, and the technique only treks you that it is over that age.