Have someone pick a card in a shuffled deck of 52 cards, ask him to put it back in the pack.

Put on the Spex glasses and give him a lighter, once the flame flights up, you're able to reveal the card!

Perhaps, your spectators would like to see this experiment once more, ask a second spectator to select a different card, give the glasses to your first spectator and as you light the flame, he is not going to believe what he sees.