1. due to the gravitational pull of the moon and of the sun on the earth.

2. due to gravitational pull waters of the oceans are attracted towards moon. Water being liquid rises higher than the solid crust of the earth. This is known as the high tide. When the level of the water falls, it is known as low tide.

3. due to the rotation of the earth tides occur twice a day at a place.

One tide occurs on the earth surface facing moon and the other tide occurs at the opposite side of the earth.