Tide is the regular rise and fall of ocean water levels that happens roughly twice a day. When the tide comes in, water creeps up the beach, covering sand that was dry hours before. When it goes out, the water retreats, revealing tide pools full of sea creatures, shells, and seaweed.

Tides are caused by the gravitational pull of the moon and sun on Earth's oceans. As the moon orbits our planet, its gravity tugs at the water, creating bulges in the ocean that we experience as high tide. Between these bulges, the water level drops to low tide. The sun contributes too, though its effect is weaker because it's much farther away.

Understanding tides matters enormously. Ship captains time their harbor departures for high tide, when the water is deep enough. Fishermen know certain fish bite better at specific tides. People who explore tide pools must pay attention, or they risk getting stranded when the tide rushes back in.

The word also describes powerful movements or trends: a tide of excitement might sweep through a crowd, or a coach might talk about turning the tide of a game, meaning shifting momentum from losing to winning.