You throw a ball into a box and close the lid. You hear the ball bouncing around inside as the ball's energy changes from gravitational potential energy to kinetic energy, to elastic potential energy, and so on. If you wait a minute or two, what will have happened to the ball's energy?

Thermal energy.

Explanation:

We know that energy is conserved and it only transform one form to another form, Like potential energy transform in to kinetic energy when a ball fall down from a height and obtain some velocity.

In this case energy changes from gravitational energy to kinetic energy then elastic energy and after some time this energy will convert in to thermal energy.

So the answer is thermal energy.