The Show:

Magician:  Have one of the guest shuffle a deck of cards as many times as he/she wants.  Ask another guest to pick one card and show it to everyone, including you.  Announce that you are telephoning Moaning Myrtle at Hogwart where she lives and that Moaning Myrtle will tell what card was selected from the deck.  Dial the telephone number and when it is answered, ask for Moaning Myrtle and hand the telephone to the person who selected the card.  A ghostly voice on the other end of the telephone tells exactly which card was picked!  Let other guests have a turn… Moaning Myrtle will always know which card was chosen.

The Secret:

Moaning Myrtle is really a secret assistant that has practiced this trick with you and will be waiting for the phone call from the party.  When the phone is answered by the assistant, say, “Hello, may I speak to Moaning Myrtle?”  The assistant will quietly and slowly start naming cards… he/she will say, “Ace… two… three… four… etc.  When the assistant says the correct number of the playing card that was chosen, the Magician stops him by saying, “Moaning Myrtle, will you talk to my friend?”  Now your assistant knows the card number.  But will the Magician is still on the phone, the assistant says, “Clubs… Diamonds… Hearts…. Spades.”  When the correct suit is named, the Magician quickly says, “Just a moment, Moaning Myrtle.”  The assistant now knows the number and suit of the card selected.  The Magician hands the phone to the guest who chose the card.  Moaning Myrtle uses a scary voice and tells the number/suit of the card and quickly hangs up.