At the cafeteria at my workplace, every time I give the cashier one of the newer $20 bills, she takes what looks like a dull yellow highlighter pen and makes a mark on the bill. I wonder if that's some special pen that's supposed to show something when she marks the bill. I've seen those a couple other places, but have never known what they are.
}:-)4
Yes, it is a special pen.
I think you can buy them at OffficeMax and places like that. There's something in the ink that makes it turn dark if the bill is fake.