You may or may not have seen this trick before. A spectator chooses a card from the deck and writes down the abbreviation on a napkin. If the Seven of Hearts was chosen, 7H would be written.
The magician then lights the napkin on fire. He rubs the ashes all over his forearms and somehow, the ashes spell 7H on one of his arms.
This trick is dependant on the setup. Cut a piece off soap off, jagged enough to use a writing utensil, but not sharp enough to hurt.
Shuffle your deck of cards and look at the top one. We'll say it is the Seven of Hearts. Using the soap, write "7H" on one of your forearms. The soap will not cause 7H to appear, but it is now imprinted on your arm. Go out into the public and perform.
For this to work, the spectator needs to choose the Seven of Hearts. To achieve this, you need to FORCE (term for making the spectator think she chose a random card when you actually made her pick the card you want) the card.
Easiest way to do this is to cut the deck in half and ask her which half she wants. If she picks the bottom, you will say "Okay, I'll put this one away." if she picks the top, you will say "Okay, we'll use this one." Bottom line, you need the top half because the top card is the Seven of Hearts.
Tell her to take the top card, whatever it is, and memorize it. Tell her to say it out loud because this trick does not involve guessing.
Because you let her choose the half of the deck, she is unlikely to question if you shuffled or not. She will say "the Seven of Hearts."
Have her remove a tissue or napkin and, with your marker, have her write down the abbreviation. When describing how to abbreviate, be sure not to say the Seven of Hearts. You want her to write the number and first letter of the suit.
(NOTE - you can have her just write down Seven of Hearts instead of 7H)
Take the napkin, fold it, and light it on fire. Tell her you need the ashes for this to work. Have her look into your eyes and repeat her card five times.
Take the ashes and rub them over both your forearms. The ashes will stain your arms, EXCEPT for where the soap wrote 7H. She will literally be staring at the name of her "randomly" chosen card on your arm.
With the right performance style, this is a heart-stopper.
Rubbing over both your forearms will make her think you are doing something randomly. Only using one suggests something may have been planned.
Fire - enough said.