There are millions of counterfeit bills in circulation. This pen allows you to detect counterfeit money before you accept it: simply mark anywhere on the bill -if it turns yellow or clear the bill is good, if it turns black or brown the bill is a counterfeit. The mark on genuine currency will disappear within 24-48 hours; therefore you are not illegally defacing the bill. This smart money universal counterfeit detector pen is effective on currency worldwide except Canadian and will not detect any series dated before 1959. 5.25 Inches long, and less than 0.5 oz. Made in USA.