A valve is a device that controls the flow of liquid or gas through a pipe or tube, working like a gate that can open, close, or adjust how much passes through. Turn the valve on your garden hose and water flows freely. Turn it the other way and the water slows to a trickle or stops completely.

Valves appear in countless places. Your heart has four valves that open and close with each heartbeat, making sure blood flows in the right direction and doesn’t leak backward. Musical instruments like trumpets and French horns have valves that players press to change which tubes the air flows through, creating different notes. Car engines use valves to control when fuel and air enter the cylinders and when exhaust leaves.

Some valves are simple knobs you turn by hand. Others are automatic, opening and closing on their own when pressure builds or drops. Engineers design valves for everything from tiny medical devices to massive water systems that supply entire cities.