A pack of cards is placed face down on the table. The magician asks for a random number, calculated by adding together the digits from the year a spectator was born. The cards are cleanly counted into a pile according to the random number. The counted to card is placed aside - all remaining cards are shown to be a mixed pack of playing cards, yet when the spectator's card is turned up, it has a Happy Birthday message written across its face.
Easy to perform and quick to reset. You're supplied with the special cards and a written instruction.