And the marker has a certain amount of potential energy as a result of its position with respect to the ground. So remember, the Earth exerts a gravitational force, and that gives it a certain amount of gravitational potential energy. Now, as soon as I let go of my marker, the marker will begin to travel downward. And so as the marker travels, its gravitational potential energy will begin to decrease. The question is, where does that gravitational potential energy actually go? Well, that energy cannot be destroyed by the first law of thermodynamics.