The energy of an isolated system is always constant.

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In an isolated system, energy cannot enter or leave the system, so the total energy remains fixed. Energy can shift between forms—kinetic, potential (like gravitational or elastic), and internal/thermal—yet the sum of all these energies stays constant. For example, a pendulum swinging in a vacuum trades potential energy for kinetic energy and back, but the total energy doesn’t change. If the system were not isolated, energy could transfer to the surroundings or into the environment, causing the total to change. So the key idea is that energy can transform internally, but the overall amount stays constant. Individual forms may rise or fall, but the total energy is constant.