system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why is Earth's core hotter than the surface? What's causing the heat?
gpt
A couple of reasons.

One is that the Earth is rather, well, big and the core has heat left over from the formation of the Earth.  It takes a long time to get rid of all that heat.  This brings us to the second reason which is that when the Earth was molten the denser material sank into the core.  This included most of the Earth's radioactive materials.  The decay of these materials keeps the core hotter than it otherwise would be.

As to why the core is hot and not the surface.  This is because the core transfers heat away from the core to the surface where the energy can then radiate into space.