The law of conservation of energy states that energy cannot be created or destroyed, only transformed from one form to another. This means that the total amount of energy in a closed system remains constant over time. For example, when you rub your hands together, mechanical energy is converted into thermal energy, which warms your hands. In practical terms, this law applies to many processes in nature and technology. In a roller coaster, the potential energy at the top of a hill is converted into kinetic energy as it descends, demonstrating how energy shifts forms while the total energy remains unchanged.