Tides are when ocean water moves up and down the coast. There is high tide and low tide. High tide is when the water moves up the coast or advances. Low tide is when the water moves down the coast or recedes. Tides are caused by the Sun and the Moon, but mostly the Moon. It is because the gravity of the Moon and the Sun pulls the water and even the whole Earth itself. High tides on one side of the Earth also make it high tide on the other side of the Earth. This is because the Moon pulls the Earth itself a tiny bit and causes the oceans to bulge on the other side too. Look at the image below. The blue bulge on the sides of the Earth represent the ocean water.