Final answer:
The cork popped off the bottle because potential energy was changed to kinetic energy.
Step-by-step explanation:
When a cork pops off a bottle, the energy is changed from potential energy to kinetic energy. The cork starts off with potential energy due to its position in the bottle, and when it pops off, it gains kinetic energy as it moves away from the bottle. This change in energy is a result of the stored energy in the cork being converted into the energy of motion.