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)
Step 2: Apply Stencil to Money
For the first bill I did, I wrote my name 'samuel' by printing out the name on a piece of paper, applying the blue painters tape around the printed letters, and then transferred the traced stencil to the bill.
For the second bill, I wrote 'FAKE' on the front of the bill. I did this by just applying the blue painters tape to the bill directly, leaving a void where the letters will be.
As you can see, the trace method worked looks better, but it took about five times longer to complete.
Step 3: Spray!
Spray each bill evenly with enough spray starch to cover all of the exposed money for about 3 seconds each.
Let it soak in for another three seconds and use another towel to wipe off the excess spray starch.
After you've wiped off the extra starch, hang them up to dry.
Step 4: Completion
Now that you're finished with the bill, you can either send it back into the wild by spending it or depositing it at your bank (if you're afraid of a clerk hassling you), or test the marker on your bill the fruits of your labor.
If you would like to see what your message looks like on the bill, just take a counterfeit money detecting marker to it.
As you can see with my bill, the areas which were covered up remained yellow (must be real money) and the areas which were sprayed turn brown (fake money!!).