Question: what makes the Earth's core as hot as it is?

Lidunka Vocadlo answered on 15 Jun 2015:

This is a big question! Well, when the Earth formed about 4.5 billion years ago, it was extremely hot and has been cooling down ever since. As well as that, the Earth’s core is hotter than, say, the surface, because it has all the mass of the rest of the Earth pressing down on it and that heats it up (something called adiabatic heating).