system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do we know the age of the earth?
gpt
Uranium dating mainly.

We find uranium deposits on earth and on meteors. We know from experiments that given a chunk of pure uranium, half will decay in 4.5 billion years. We can see the byproducts of the decay, primarily lead. If lead is mixed with uranium, we generally assume that it is there because uranium decayed to it.

We found Uranium deposits that are considered static (they don't mix/change composition beside decay), about half the uranium has decayed, a little more in fact. By this, we estimate the earth is about 4.5 Billion years old.

Edit: Misleading word deleted.