Have you ever been to the beach and built a sand castle, then watched it wash away when the water came in? Or maybe you’ve waited for the water to go out again so that you could hunt for seashells. The rise and fall of the water line along ocean coasts are known as tides. In most places, high and low tides happen twice a day. The reason this happens is because of gravity (GRA-vih-tee).
Gravity is a force that pulls objects together, kind of like a magnet. The Earth has gravity, but so do the Moon and the Sun. The Moon affects tides the most. As the Earth rotates, or turns, the force of the Moon’s gravity pulls on it. That force is different on different parts of the Earth’s surface. We don’t see the effect of the Moon’s gravity very much on land, but as the Earth rotates, we can see the effect on our oceans.
The Moon's gravity pulls everything on Earth a little bit towards the Moon. This force also squeezes the Earth, and it moves the water of the oceans. Picture squeezing a ball in your hand: the ball bulges out at the front and the back. Now picture the Earth being squeezed and rotating. As it makes a quarter turn away from the Moon, the same part of the ocean that was at the bulge in the ball is now getting squeezed. That’s low tide. Another quarter turn and we’re at the far side, which is the other bulge that appears when you squeeze the ball. That’s high tide.
Watch this video from PBS with a grownup—it's a little complicated, but it has a lot more information!