Unplugging a clogged sink with a plunger is easy, once you understand the principle behind how a plunger works. Pumping up and down does no good if you don't block the overflow because all the pressure from the plunger just moves up the double wall of the sink and out the overflow. No pressure is put on the clog. Stuff a rag into the overflow and hold it there with one hand. Now the second secret is that the plunger pushes water to the side in the down stroke, which doesn't put much pressure on the blockage. In fact, putting pressure on a blockage might make it block even more. The key is the up-stroke. On the way up, the plunger creates a good suction under the cup, pulling the blockage back into the sink. A blockage will have less resistance to coming back than to going forward. That will break up the blockage and everything can flow away.