"For over a hundred years," you tell your participants, "fingerprints have been a reliable method for making a 100% accurate identification. Now they're called 'biometrics' and are used to cash checks, open doors, and even to protect data on cell phones and personal computers."
As you riffle through a shuffled deck, a participant is asked say "Stop" at any time. When he does, the deck is broken at that point and the upper half lifted so only he can see the card on the face of the upper block of cards.
"Give me a thumbs up hand gesture, please," you request, "and place your thumb directly on the index at a corner of the card where both the value and the suit can be covered by your thumb. Now we need a way to see and process fingerprints. That's where technology lends a hand."
You displays small piece of electronic gear, about half the size of a deck of cards and not quite as thick, with a computer USB connecter extending from one side. You continue, "Absent a desktop computer, we'll use this in stand-alone mode, it scans and examines fingerprints."
With a push of a button the device turns on, displaying a small illuminated scanning surface where a finger or thumb can be placed to input it's biometric image. You instruct the participant, "Please use the same thumb with which you marked the card to register your thumbprint on the device." He complies and notices the printscan on the small screen.
As your participant holds ScanTastic, parts of his fingerprint seem to fade. Within fifteen seconds the only bits of the thumbprint that remain form a number and a letter - perfectly identifying the selected card.
The effect can immediately be repeated with a different selected card - again and again and again if you wish. Get two or three people each to take a card at the same time, then get them to pass ScanTastic to one another to register first their thumbprints and then their surprise when each card is precisely identified!
The detailed, illustrated instructions make ScanTastic a snap to perform. ScanTastic is ideal for table-hopping because there's no setup involved and if the folks at adjacent tables compare notes, they'll discover they all chose different cards!