A counterfeit detector pen contains a solution of iodine. This iodine reacts with the starch molecules that are present in a normal, wood-based paper, to leave a dark brown mark. This black mark indicates that the bill is fake. Neither linen nor cotton fiber will react with iodine, so the mark will not turn black.

Do counterfeit pens really work?

Counterfeit detection pens are not always accurate and may give you false results. ... A counterfeit note cannot be exchanged for a genuine one, and it is illegal to knowingly pass counterfeit currency. If you live in the United States and you think you've received a counterfeit note, immediately notify the local police.

What color does counterfeit money turn with pen?

The pen reacts to starch contained in most paper sold around the world. Real U.S. currency paper does not contain starch. So if the bill is real, the ink turns yellow. But if it's fake, it will turn a dark blue or black.