Here's a breakdown:

* Radioactive Decay: The Earth's interior contains radioactive elements like uranium, thorium, and potassium. These elements naturally decay, releasing heat energy in the process. This is the dominant source of heat within the Earth.

* Residual Heat from Formation: When the Earth formed billions of years ago, it was a hot, molten ball. Some of this original heat is still present in the Earth's interior, but it's gradually dissipating.

* Friction: The movement of tectonic plates, along with the convection of the Earth's mantle, generates friction which contributes to the overall heat within the crust.

While the sun provides heat to the Earth's surface, its energy doesn't penetrate deep enough to significantly impact the heat of the crust.