From the The Houston Chronicle:
While on its mission to provide free information, the Houston Public Library has to swat at the occasional counterfeiter.
At each of Houston’s 42 public library sites, equipment at the circulation desk includes a counterfeit detector pen.
Widely available at office supply stores, the pen contains a solution that detects bills printed on common copier paper rather than the cotton-and-linen-fiber paper the U.S. government uses.