The earth and the moon pull each, other due to the gravitational force of attraction. Water on the earth (ocean) is attracted by both the earth and the moon. The molecules in water are not rigidly fixed. Therefore, when the moon and the sun both lie on the same side of the earth (new moon) or on the opposite side of the earth (full moon) with the ocean, the combined gravitational pull causes a bulge in the ocean water. This bulging out of the ocean water in the direction of the moon is called tide.
