A tap that refuses to turn off is usually caused by the failure of a small internal component that is responsible for creating a watertight seal. In traditional "pillar" or "cross-handle" taps, the most common culprit is a worn-out rubber washer. Over time, the constant pressure and friction of turning the tap on and off cause the rubber to perish, crack, or flatten, allowing water to seep through even when the handle is tightly closed. In modern "mixer" or "quarter-turn" taps, the issue is often a faulty ceramic disc cartridge. If the ceramic plates inside the cartridge become cracked or if grit and mineral scale get trapped between them, the seal will fail. Other potential causes include high water pressure that forces its way past the valves, a stripped thread in the tap handle that prevents it from fully engaging with the valve, or significant mineral buildup from hard water that physically obstructs the mechanism from closing completely, leading to a constant and wasteful flow of water.