system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can someone define the age of something? (mummy, dinosaur's bone, etc) I just read an article "Baby bird discovered in 99-million-year-old amber"
gpt
There are various dating methods, and they can be checked against each other to make sure the results are consistent. The age of fossils is usually determined by radiometric dating. How it works basically is that there are a bunch of elements that occur on Earth naturally, and a bunch of elements, or different isotopes of certain elements, that only exist on Earth as a result of one element radioactively decaying to the other element. The decay always happens at a constant rate. Based on how much of the source element and the result element are present in the sample, we can evaluate how old it is.