system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is the Earth's core still hot 4.5 billion years into its existence?

I mean, I get that it's pressurized and all, but how does it maintain enough heat to keep iron melted?
gpt
The earth is actually still heating itself. We're not just a bunch of rock, the core and mantle actually have atoms breaking down, releasing heat. We are effectively sat on a nuclear reactor set to slow.

Early earth was hotter, and has lost a LOT of heat over time, but the amount we have left isn't just here form the start, our planet is effectively burning fuel to keep active.