You offer a bet of $100 (taking a $100 bill out of your wallet and placing it on the table) that you'll accurately predict a card chosen at random by a volunteer from the audience. You lay the Mega-Card (11 x 15 inches) with your prediction face down on the table.
You then ask the volunteer to select a card from a regular pack - and then, you offer to up the odds by making your own conditions much tougher!
You proudly announce that your prediction will be exactly HALF of the spectator's card, saying "If you've selected the 4 of Diamonds, I'll have the 2 of Diamonds. If you've selected the 8 of Clubs, I'll have the 4 of Clubs".
Then you ask the spectator to look at his selected card without you seeing it, and to show it to the audience. They see the 7 of Clubs and start laughing, as they know you can't possibly have the half of it.
But lo and behold - you turn over the Mega-Card to expose the 31/2 of Clubs... and take back the $100 bill. LAUGHS and APPLAUSE. (Probably the highly disappointed volunteer is not laughing..)
This is a beautifully designed and produced version of the well known "sucker" effect.
You have to force a card.
All the rest is in the amusing dialogue and the Mega-Card.