The cooling of the earth and why the core is hot is actually a very interesting topic. Lord Kelvin (who has the temperature unit named after him) came up with a

The cooling of the earth and why the core is hot is actually a very interesting topic. Lord Kelvin (who has the temperature unit named after him) came up with an argument that the earth had to be 20 to 40 million years old if you assumed it started as a molten lump of rock and solved the equations for how long it would take to cool. (This age for the earth made both the evolutionists and creationists unhappy, by the way.) He had a very clever argument, but the factor he was missing was radioactive decay, which provides an additional heat source. He also carefully figured that the Sun couldn't be more than 20 million years old, but here he was missing the existence of fusion as a power source.