A sluice is a channel or gate that controls the flow of water. Picture a wooden gate built into a dam: when you lift it, water rushes through; when you lower it, the water stops or slows. Miners during the California Gold Rush used sluices constantly, directing water through long wooden troughs to wash gold from dirt and gravel.

The word captures both the structure itself and what it does. A mill might have a sluice to control water flowing to its wheel. A canal uses sluices to manage water levels. When water comes pouring through forcefully, we say it sluices through, like rain sluicing off a metal roof or waves sluicing across a ship's deck.

Sluices gave people remarkable control over water, letting them direct it where they needed it and hold it back when they didn't. A farmer might use a sluice to irrigate fields at just the right time. A town might use one to manage a reservoir. The simple mechanism of raising and lowering a gate to control a powerful force has made sluices essential tools for thousands of years, and they're still used today in dams, canals, and water treatment systems.