system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how do we know dating something is accurate? If the materiales from which something is made are millions of years old, what difference does it make once it’s made into something?
gpt
The radioactive isotopes that are checked for with radiometric dating exist naturally in the world. The acts of molten rock churning in the Earth, or the waters mixing in the oceans, or animals eating plants, and even breathing, all increase the amounts of the isotopes found within the body.

Once the thing is no longer moving, eating, or breathing, it is no longer adding the radioactive isotope into itself, and it begins to decay.

The most common form of radiometric dating is carbon dating, which is most useful for recent (within a few tens of thousands of years) artifacts made of living material, like cloth or pottery. That carbon isotope is generated by sunlight, which replenishes the amount found on the planet.