If you stood on any of Jupiter's moons, Jupiter would always stay at the same place in the sky: every moon is tidally locked to the gas giant. So how could there ever be tides? The answer is that the orbits of the moons aren't perfect circles: Jupiter changes size in the sky over the course of the moon's orbit. This means that Jupiter's gravitational attraction on the moon changes, as does the relative gravity across the moon's diameter. When Jupiter draws close, the tides tend to expand the moon, and the moon would break apart if it wasn't a solid object. Still, the result is that Jupiter's fluctuating gravity heats the interior of the moon, with the strongest effects on the closest and biggest moons (Io and Europa). This results in volcanoes on Io and possibly liquid water on Europa.