Why does the ocean have tides?
For grown-ups: the science
The Moon's gravity pulls ocean water into moving bulges, making high and low tides.
Tides are caused mainly by the Moon's differential gravitational pull across Earth, which creates tidal bulges on the near and far sides. As Earth rotates, most coasts pass through these bulges and experience changing water levels; the Sun also contributes to tides.
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Transcript
The water at the beach can reach far up the sand, then slide back. Why does it do that?
The Moon pulls on Earth with gravity. Gravity is a pull that also tugs on ocean water.
Water bulges a little toward the Moon and a little on the other side of Earth. These bulges move as Earth turns.
When a bulge reaches your beach, the water is high tide. Between bulges, the water is low tide.
So tides happen because the Moon's gravity pulls on ocean water while Earth turns.