Tide

Tides are the rise and fall of water level in the ocean. Twice a day, the ocean gets higher and then it gets lower again. This happens because the moon's gravity pulls on the water and makes it bulge out in the direction of the moon. The bulge of water on the side closest to the moon is the high tide, and the bulge on the side away from the moon is the low tide. The tides also change with the phases of the moon, which is why you may have seen higher than average high tides during a full moon.