Explain Like I'm Five | Don't Panic!

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The original was posted on /r/explainlikeimfive by /u/dreamweaver2070 on 2023-08-12 00:59:09.

I'm aware how radiometric dating works in general and that, for example, the half-life of uranium to iron is about 4.47 billion years. What I don't understand is how scientist determine that is the correct amount of time it takes for half of the uranium atoms decay to iron. How do we know it's not actually 20 million years or 600,000 years? Please explain to me like I'm five!