Roller coasters
This ball is rolling on a track that has been bent into a complicated shape, with two valleys. The ball is released at one end. Will it run off the other end? Not if the two ends are the same height! We start the ball with a certain amount of potential energy, determined by how high you lift it to start. Some of the ball's potential energy turns into kinetic energy as the ball rolls into the valley; but as it ascends the other side, the kinetic energy is turned back into potential energy. It can't ever go higher than it started, because it doesn't have enough energy.
This also explains why trucks on mountain roads go fast in the valleys and slow at the hilltops: the trucker is not using the brakes on the downslope so that he won't have to use the engine on the upgrade. He's obeying the law of conservation of energy! Unfortunately, this may involve breaking the laws for motor vehicles.