Businesses have relied on the Dri Mark Counterfeit Money Detector Pen for over 25 years - and you can too. This efficient, reliable tool helps prevent loss associated with counterfeit U.S. currency. It's simple to use: simply mark the bill presented to you, and if the ink stays yellow, it is authentic U.S. tender. If the mark turns black, the bill is counterfeit. It's fast, accurate and extremely easy to incorporate into daily transactions.