It Knows When You Are Lying!

You "inject" a volunteer with a new, powerful Truth Serum. No matter what, your volunteer will now have to tell the truth. To test the effectiveness of the serum, you produce a deck of cards, one of which is selected by your volunteer and shown. The card is then returned to the deck, which is then cut several times. 

You begin to peel off one card after another and ask your voluntreer when the selected card shows. You volunteer may or may not be telling the truth. You have no way of knowing. (That's where the test comes in!) You remove the card named by your volunteer and place it face-up between his or here hands. You administer the "oath" in which you  volunteer swears to tell the truth and nothing but the truth. Now, turn the card over. There, printed on the back of the card is large letters is " YOU ARE LYING!" Also written there is the name of the card selected.