When a moon or planet is between two larger bodies, such as a star and a large planet, or even two very large moons, the large bodies pull at the little one with their gravity. The smaller body can't escape, and cannot fall toward either of the larger bodies, so the pulling puts stresses on the smaller body.

If a moon is near a planet, the gravity of the planet and the sun could pull the moon this way and that. Sometimes the gravity pulls on the moon in opposite directions. At other times, the pulling might be from similar directions. The gravitational pulling causes stresses which cause friction in the rocks. The rocks, of course, move only a little, but that is enough to generate heat. The heat may cause lava to come out, or even water, if the planet is covered with ice. Scientists think that they have seen evidence of ice volcanoes on Titan, a moon in our solar system.

This little animated picture shows an exaggerated view of the pulling and squeezing that could result. Of course, the distortion would be much, much less. However, you can see that the crust of the planet would experience some distortion -- and lots of earth quakes!

If the moon or planet were covered by a frozen ocean, ice near hot rocks might melt deep under the surface, providing liquid. If living organisms evolved here, they would live in darkness, getting warmth and chemicals from geothermal processes. Some organisms that live on geothermal energy have been found near volcanic vents deep in the oceans of the earth.  Scientists have even found bacterial living in rocks underground on our planet!

Who knows what life forms may exist on other worlds?