Illegal?

As my husband pulled this bill out of his wallet recently, he noticed the "In SCIENCE we trust" stamp and showed it to me. I always thought it was illegal to deface American money, but he disagreed. Of course, I had to look it up. Title 18, Section 333, of the Unites States Code states: "It is unlawful to mutilate, cut, deface, disfigure, perforate, or otherwise damage drafts, notes, or other evidence of debt that has been issued by a national banking association with the intent to render the bill, note, or other evidence of debt unfit to be reissued." 

So, as long as the intent isn't to make the bills unusable, stamping messages on U.S. money is perfectly legal. Looking at the list of no-nos (mutilate, cut, deface, disfigure, perforate, and otherwise damage) it makes me wonder - just how much disfigurement would it take to make a bill no longer fit to be reused? Not that I'm going to try, however. If a person is found guilty of this crime, the punishment is a fine and up to six month in jail, though proving you had intent might be tricky.