Janus and Epimetheus are co-orbital moons of Saturn. They share almost the same orbit around Saturn, at about two and a half Saturn radii. One of them is a tiny bit closer to Saturn, so it moves just a little bit faster. Once every four years the fast one laps the slow one, overtaking it from behind. When that happens, the two satellites exchange orbits: the slower one becomes the faster one, and vice versa.