You fit the small key into the winding hole at the back of your mechanical wall clock. You turn it clockwise, feeling resistance build as the internal spring tightens. A few full rotations, and it's fully wound.
Immediately after, the clock ticks steadily. The pendulum swings smoothly, and the hands advance across the face just as before. No change is apparent in its operation.
Days pass. Each morning, the clock displays the correct time. The ticking continues without falter. It performs reliably, showing no hint of difference from before the winding.
Then, one quiet morning, the sound stops. The pendulum hangs still. The hands remain fixed at a specific hour and minute. The clock has ceased running.
This halt occurs because the wound spring released its energy gradually to power the gears and escapement. That process stayed hidden while the clock ran evenly. The outcome appeared only after the full interval, separating the winding from the stop.