You thought about the Jack of Clubs? That’s good! This is the only card in the deck: only ONE Jack of Clubs because the other 51 cards are… the 3 of Hearts!
You will be able to show all the cards of your deck on the table and show the spectator that this one was in fact made up only of the 3 of Hearts (for example) except one card: the Jack of Clubs, the card thought of by the spectator…
In this mentalism trick, you can easily choose the atmosphere you want to use — all is possible!