pen will tell you whether a banknote is fake or genuine in only 2 seconds! Simply mark the banknote and watch: if the ink turns dark, the note is likely a counterfeit. The iodine-based ink reacts with a starch found in all standard papers, but not in the official cotton-based banknote paper used by central banks worldwide. What’s more, it helps take fake money out of circulation: the pale ink will fade away on genuine banknotes, but leave a permanent purple mark on suspected counterfeits.