Why is the center of the Earth still hot?

Hi, Matt:

I was wondering why the interior of the earth is still red hot. It would seem that after four billion years, it would have cooled off by now.

-- CH, San Diego

Pick one: (A) Earth's core is a solid moon-sized ball of iron spinning in an envelope of roiling, gushing molten iron, which remains molten because of the heat of radioactive decay and pressure, all a mere 25 miles under our feet; or (B) The center of the earth is made of pizza cheese, which, as everyone knows, never cools off.