The cause of tides on the Earth is the moon. As it orbits our planet, its mass exerts a gravitational force on our planet, pulling objects towards it. But while solid objects aren't (significantly) deformed by that, liquid ones can be - and the oceans are.
The effect is that the oceans are pulled into an oval around the spherical Earth: slightly towards the moon overhead (high tide), with a corresponding peak on the other side of the world. Meanwhile there is a dip (low tide) at the "horizon" in the middle. As the moon moves, so does the point of low and high tide. Effectively, as the Earth rotates, every 24 hours the moon appears in a different point in the sky - so the times of the tides change.
The change in sea level may not seem remarkable, but tidal effects are enormously powerful. The amount of sheer energy required to shift an entire ocean is colossal - hence various attempts to harness this energy with tidal barrages, for example in Cardiff Bay.
In a sense, that energy input is the reason why scientists looking for life elsewhere in the solar system are very interested by Europa, an ice- covered moon of Jupiter.
Scientists reckon that when it formed, Europa was composed of a mixture of rock and ice. As this coalesced, says Stephen Squyres, of Cornell University, New York, the tidal effects of orbiting the huge mass of Jupiter, allied to tidal effects from its two neighbouring moons, would cause tidal flexing - heating up the core. That would melt the ice.
Now, billions of years later, Europa should be an icy crust with an ocean beneath, all heated by the core - whose only energy input is tidal effects.
Observations by the Galileo spacecraft confirm this: there are no craters, as would be expected if the moon were solid ice all the way through. Instead, craters seem to close up soon after forming, like ice remelting on a pond.
Why do they think there's life? Because on Earth, even in the deepest oceans you find life forms when there's a hot source, such as deep thermal vents from volcanoes. If the tides inside Europa are forceful enough, there should be something down there. Eugene Shoemaker, the astronomer, who died last year, said: "If I really had to choose a place to look [for life], and it was between going to Mars or to Europa, I would say Europa." The next mission should head off in 2001, to arrive in about 2005.