A mass bobs back and forth about its equilibrium point, pulled always toward centre by a restoring force. Watch energy slosh smoothly between kinetic and potential — fastest at the middle, frozen at the turning points — while the period stays stubbornly fixed.
Try this: raise the mass and the block slows down — the period grows as √m while amplitude and total energy hold steady. Nudge damping above zero and the swing decays, its envelope shrinking as e−γt.