Which principle states that energy cannot be created or destroyed?

The law of conservation of energy is a fundamental principle in physics, which asserts that the total energy within a closed system remains constant over time. This means that energy can neither be created nor destroyed; it can only be transformed from one form to another. For instance, when you rub your hands together, the mechanical work done through the friction is converted into thermal energy, or heat. Likewise, in a closed system, the energy can change forms between potential energy, kinetic energy, thermal energy, and so forth, but the overall amount of energy is preserved. This principle is foundational in various fields of physics, from mechanics to thermodynamics, and plays a crucial role in understanding energy transfer processes in any physical system.