When water is pumped into a skimmer it is met with a stream of fine bubbles. The theory behind this, is that protein based material (decomposing material) will stick to the outside of each bubble in the form of a slimy protein coat, these bubbles fight their way to the top of a "stack" of bubbles. As these bubbles rise inside the column of the PS they rise towards a venturi (in most cases). As they rise they get closer to the top rim of the venturi, this is located inside the skimmer cup. Once it reaches this rim it over flows all of the coated bubbles into the cup. This process extracted decomposed matter, and waste from the water and into the collection cup on the top of the skimmer.