Designed to detect counterfeit currency, this detector pen works on the majority of counterfeit US bills. Simply place a small mark on your bill and watch as the sensitive ink works instantly in detecting the counterfeit bill. The dark ink will suggest the bill is fake while a lighter ink will determine the bill safe. Once the ink has dried, the mark will fade and disappear. Size is 5 1/4" x 2/3" x 5/6"