Cost-effective and easy to use, money-checker pens with special color-changing ink are essential tools for quick visual counterfeit detection. The specialty ink formulated for use with US currency only. To use, simply mark a small line anywhere on the bill. If the mark is amber (light brown or yellow), the bill is genuine. If the mark is dark (brown or black), the bill is suspect. Best when used in conjunction with other counterfeit detection methods.