Jupiter's moon Io is the most geologically active world in the Solar System. Its internal heat is generated by the friction of changing tides, heat that rides across the surface in lava flows and is blasted to orbital heights through a network of 400 volcanoes. The same internal processes are operating on another of Jupiter's moons—Europa—but its heat may go into warming a subsurface ocean.
Tidal heating results from a shifting gravity differential between a moon's far side and its near side. The strength of gravity always diminishes as the distance from its source increases, but this discrepancy is negligible for moons in distant orbits around low-mass planets. Io is about the same distance from Jupiter as our own moon is from Earth, but Jupiter is about 318 times as massive as Earth. Under these conditions Io feels tremendous gravitational pull from Jupiter—it swings around the planet in only 1.7 Earth days. The pull from Jupiter's gravity, though, is stronger at the side of Io closest to Jupiter. This effect causes Io to deform slightly from a sphere into an egg-shaped form called an ellipsoid.
A static gravity differential will not cause tidal heating. The internal heat in Io is caused by a changing gravity differential. Since Io's orbit is not circular, the differential of Jupiter's gravity will be more significant at closer orbital distances from the planet; Io's ellipsoid shape will thus be more pronounced when the moon is passing close by Jupiter. The bulges of this moon wax and wane, creating frictional heat in the same way a paper clip will become warm if it is flexed a few times. This interior heat reaches the surface through volcanism and is then radiated away into the cool and refreshing cosmos.