The Sharpie Counterfeit Detector Pen uses specially-formulated ink that turns dark blue/black when money is fake. When marked on a bill, the ink in this counterfeit money detector reacts to paper other than the special paper used by the Treasury Department. Each counterfeit bill detector can be used up to 2,000 times. This counterfeit detector is a must-have for any business dealing with money. The technology reacts to the presence of a key ingredient present in papers that are not used by the U.S. Treasury.