Perc is an abbreviation of the word percolator. Percolators use water to filter smoke by breaking up the smoke stream, removing tar from the smoke, and forcing the water and smoke into close quarters. Percs come in a variety of sizes and styles. Some percs are better for screening tar and ash. Some percs are better for cooling the smoke before it reaches the mouthpiece, but the fundamental mechanics of all percs are largely the same.