Final answer:
As a rock falls off the side of a cliff, its potential energy is converted into kinetic energy. This means that its energy changes from being stored in the rock's position to being stored in its motion.
Step-by-step explanation:
When a rock falls off the side of a cliff, its potential energy is converted into kinetic energy. Potential energy is the energy that an object possesses due to its position or condition, in this case, the rock's position on the cliff. Kinetic energy is the energy an object possesses due to its motion.
As the rock falls, its potential energy decreases while its kinetic energy increases, until it reaches the ground and all its potential energy is converted into kinetic energy.