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human
Eli5 - How does earths inner core stay hot?

Today I was reading about how it is cooling and its anticipated rate.   And that it should still stay hot for millions of years or more.    My question, if oxygen is one of the things needed for heat/fire.   How does the inner core stay hot, since it seems like it should be deprived of oxygen, for the most part?
gpt
The Earth’s core is hot, but it’s not on fire. It doesn’t need oxygen to stay hot. The earth’s crust and mantle are like a celestial thermos flask, which is doing a very good job of keeping the core of the planet from losing the heat which was generated when the planet was formed.