Flapper
The rubber seal at the bottom of a toilet tank that holds water in until you flush.
The flapper is the hinged rubber (or silicone) seal covering the flush valve at the bottom of your toilet tank. When you flush, the handle lifts it by a chain so the tank empties into the bowl; then it drops back to seal until the tank refills. Because it sits in standing water, the rubber slowly warps, stiffens, or builds up mineral scale — and a flapper that no longer seals is the single most common cause of a [running toilet](/guides/toilet-keeps-running), silently leaking water from the tank into the bowl. The EPA recommends replacing it at least every five years. Swapping one is a five-dollar, ten-minute job — see [how to replace a toilet flapper](/guides/how-to-replace-a-toilet-flapper).