the law of conservation of energy
Where does the energy go when a bouncing ball finally stops, or a car brakes to a halt? It is not destroyed — it has changed disguise, into heat and sound. The law of conservation of energy is physics' grandest piece of bookkeeping: energy can be transformed from one form to another, but it can never be created out of nothing or made to vanish. The total always balances.
Stated carefully, the total energy of an isolated system (one nothing crosses into or out of) stays constant over time. Energy freely converts among its many forms — kinetic, potential, thermal, chemical, electrical, nuclear, radiant — but if you add up every form before and after any process, the two totals are identical. This is broader than the conservation of mechanical energy: even when friction wrecks the mechanical total, the energy simply reappears as heat, and the full sum is untouched.
It is one of the deepest and most tested laws in all of science, with no known exception. In heat and thermodynamics it becomes the first law of thermodynamics. It is also why a perpetual-motion machine that makes free energy is impossible: you can never get more energy out than you put in, because there is no hidden source for the books to draw on. Every apparent 'loss' is just energy you have stopped keeping track of.
Trace the energy in a bouncing ball: chemical energy in your arm becomes the ball's kinetic energy, which becomes gravitational potential energy at the top, back to kinetic on the way down, and at each bounce a little turns into heat and sound in the ball and floor. Add every form and the total is the same throughout — the ball just stops bouncing once all of it has become heat.
Nothing is lost — the bounce dies only because the energy has quietly all turned to heat and sound.
Mechanical energy alone is often not conserved (friction sees to that), but total energy always is. Saying energy is 'lost' to friction is loose talk: it is transferred to heat, not destroyed.