Jumbo Brainwave Deck (4×6)
The magician puts a deck of cards on the table closed into the case. A spectator freely thinks of a card and name it. The magician takes the deck opens it in his hands and shows how a single card is turned on the back. Clearly this is the card named by the spectator.
An effect of big impact on the public because the card is freely thought. The spectator can name any card in the deck.