What are the two sources of heat in the Earth's interior?

There are several main reasons for the heat inside the earth. Keep in mind, though, that scientists really don't agree on how hot the core of the earth might be. 3000C? 4000C? 7000C? One source is radioactive decay. Elements including Uranium and Radium are constantly decaying and releasing energy. A second source is simply the result of Earth's formation -- billions and billions of tons of rocks and metal and gasses compressing downward release a lot of heat. The last reason is tidal stress. As the moon goes around earth, its gravitational field makes shifts in earth's crust and mantle. These shifts produce heat. Some scientists think that these models still don't adequately explain the continued tremendous heat in Earth's core, but these ideas are the best we have for now.

-MathTeacherGuy, Santa Clara, California.