A test-condition impossibility! When the magician notices that he's missing a few cards from his blue-backed deck, a few cards are randomly chosen by a spectator from a red-backed deck and added to the blue deck. When the magician discovers the missing blue-backed cards, they match the red-backed cards chosen by the spectator! Different cards can be used every time the effect is performed. A self-working killer effect from David's Magic Castle Act.