Pen Detects Counterfeit Money

We've all seen it used. A pen that the clerk in the store writes on our money with. What's going on? The pen has specially made chemicals that either turn light brown or dark black when writing on a bill. If the line is dark black the bill is a counterfeit. If it is light color then the bill is OK. These pens are available for purchase on the internet. Checking this way makes us all safer. With the new digital printers, scanners, and computer software, no document is immune from imitation or counterfeit. I work hard for my money and don't want anyone to give me fake money for change. The counterfeit detection pens that make the most sense to me are the ones that have a small coil attached and a sticky pad to hold the pen in place. I don't want my checking out at a store to be delayed while the clerk fumbles around searching for a pen to check the bills. I would guess that a pen like this would be worth two of the pens without coils and sticky pads just for customer convenience. Also, how many of those pens do you figure go home at night in the pocket of the clerk because there is no coil holding it to a sticky pad? I think it's worth it. Keep on scribbling away and stop the bad guys from passing any bad money to me. - gg