Choose a number card from a deck of cards.
Take a small piece of a regular bar of soap. Put just a little water on the soap and write the number on the card that you have chosen on your arm.
Use a force card trick on the audience member to get him to choose the card that you want him to. Have him pick this card, memorize it and place it back into the deck and then shuffle it in.
Give the audience member a piece of paper and a pencil. Have him write down the number on the card he chose from the deck and then crumple the paper.
Lay the paper on an inflammable surface and light it with a lighter or match until it burns to ashes.
Ask the audience member to rub the ashes on the arm where you have the soap.
Watch your audience member gasp as the number of the card he chose magically appears on your arm in the ashes.