Tides are caused by the gravitational pull of the moon and, to a lesser extent, the sun on bodies of water. The earth's water bulges/moves toward and away from the moon(/sun), while it becomes shallower in areas perpendicular to the pull. Thus, there are two complete tide cycles per day.
Tides occur mainly in oceans because that is basically one huge body of water that is free to move all over the earth. Lakes and rivers do not cover enough area to have their water be moved significantly by gravity, or in other words, to have tides.
Justin Clifford, High School Student, Alpine, Utah
'One cannot help but be in awe when he contemplates the mysteries of eternity, of life, of the marvelous structure of reality. It is enough if one tries merely to comprehend a little of this mystery every day.'