Eight Ball by Emerson and West

Five blank cards and one 8-Ball card are shown on both sides. Then spectator is then asked to choose one of the face down cards and chooses the only odd card, the 8-Ball! He chooses again, and without looking, you inform him that he again has chosen the odd card! However, the card is actually blank.

When the "error" is called to your attention, you show that the card is in fact the odd card, because the other five cards have magically changed to 8-Ball cards.

This classic packet trick from Emerson & West is easy-to-do, and now available printed on Bicycle cards!