Why not have some fun and leave a secret message on the bill for the clerk to discover.
First I should explain how the counterfeit money detecting markers work.
These markers are basically just a marker filled with iodine. When you write on a bill, the iodine either turns yellow or brown based on the amount of starch in the paper.
The US paper currency does not have any starch in them, so if you write on them with one of these markers, the line turns yellow. Normal paper does have starch in it, so writing on a counterfeit bill will result in a brown line.
The marker will fail if you have counterfeit money which doesn't have starch in the paper or you have real money with starch in it (or on it)
The idea of this project came from the magician James Randi. For the lulz, he says, "I regularly go to my bank and take out a couple thousand dollars in $50 bills. I bring them back to the JREF library, lay them out on the large table there, and spray them with canned spray-starch. I let them dry, then turn them over and treat the other side. When they're all prepared, I put them back in the same wrappers, make out a deposit slip, and trot out to my bank again."
I wanted to add onto this by writing message on the bill, rather than just make the counterfeit money detecting marker break.
Step 1: Supplies
1. CA$H (preferably new crisp bills)
2. Spray Starch (from Walgreen for ~$1.50)
3. Blue Painter's Tape
4. Dri Mark Counterfeit Detector Pen (only if you want to test your work)
(Image of money by AMagill)