A plunger is a device that is used to release stoppages in plumbing. The tool consists of a rubber cup with an attached stick "shaft", usually made of wood or plastic. A different bellows-like design also exists, usually constructed of plastic. For the common plunger, the cup is pushed down against the drain opening, and either pressed hard into the drain to force air in, or is pushed down until the rubber cup is flattened, then pulled out, causing a vacuum that attracts material. The intent is to loosen or break up a blockage caused by excessive material in the drain.
A kitchen plunger looks like a ball cut in half while a toilet plunger looks more like a distorted ball with a large hole on the bottom.
A plunger is much more effective when there is water in the pipe because water does not compress and will thus transmit more of the applied force than air. When a plunger is ineffective, it is often supplemented by a chemical drain cleaner in the case of sinks and tubs, or in cases of main line, toilet or stubborn clogs, a plumber's snake. 
"DIY - How to unclog a toilet using a plunger ".
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