If you stand on a beach long enough, you’ll see the sea slowly rise up the sand, then slowly retreat again. That’s the tide. And the reason it happens is the Moon, 240,000 miles overhead, gently pulling on the ocean with its gravity.
As the Earth spins, the side of the planet closest to the Moon has its oceans slightly pulled up toward it, creating a bulge of high water - high tide. There’s another high tide bulge on the opposite side of Earth, where the planet itself is pulled slightly away from the water. The two low tides sit in between.
The Sun also pulls on the ocean (though less strongly, because it’s much farther). When the Sun and Moon line up - at new moon and full moon - their pulls combine and produce especially big tides, called spring tides. In some places, like Canada’s Bay of Fundy, the tide can rise more than 50 feet between low and high.
