The moon exerts a gravitational force on the Earth, which causes the tides. The gravitational force of the moon pulls on the water in the oceans, causing it to bulge out towards the moon. This bulge of water is called a high tide. On the opposite side of the Earth, the water also bulges out, away from the moon. This is also a high tide. Between these two high tides, there are two low tides. The low tides occur when the water level is at its lowest point.