FIRE INK by Marcos Cruz

stmagicbox.com...
Fire Ink is an idea by Marcos Cruz.
You choose a card from your deck, then from your wallet you take out a bill (dollar or euro) you heat your fingers with the help of a lighter, the heat of your fingers will melt the ink on the bill dragging it towards a end and the digits of the selected letter appear in the drag.
It is very visual and you can enjoy it with your audience.