State-of-the-Art Bogus Money

I gave a clerk a $20 bill for a purchase recently and he took out his special counterfeit-detecting pen and marked the bill. The marking came out yellow, meaning the bill is genuine US Bureau of Printing and Engraving work, the kind you want to have in your wallet. Like most people, I've seen clerks do this, but never has asked about it before. The store was empty besides us, so I figured I could take up a little of his time with a question.

"What happens if the bill isn't genuine?" I asked.

Instantly he obliged me by taking a small bit of newspaper -- definitely not money -- and marking it with the special pen. The mark came out black instead of yellow. The inky chemistry of that isn't something I can understand, but the results are clear enough.

"One time, I saw a bill with a yellow mark on it already," he said. "I almost let it pass, but we're supposed to mark them ourselves, so I did. It wasn't real. But it looked real and it had the yellow mark on it, so people would think it had already passed. That was clever."