As you are about to hand out your business card to a friend or client you notice that the card is blank. Reaching into your pocket you remove an interesting looking plastic case and explain that it is your portable business card printer.
You ask the spectator to sign the back of the blank card and then you place it into the printer. You show both sides of the card through the clear walls of the printer.
Now, with a snap of your fingers the blank card instantly and visibly becomes your business card with their signature still on the back!