Tell your fellow magician pal that you’ve got a new key card placement.
Have your stupid idiot victim pick a card. Proceed to go through a really fair replacement procedure. They’ll be watching like a hawk. The card genuinely appears to be lost.
“That’s it,” you say. Make a wide faceup ribbon spread on the table. “If we’re lucky, your card should now be beneath the ten of hearts.”
You look through the spread and point out the card beneath the ten of hearts. “There it is! Right there! The four of clubs! Thank you, thank you.”
The spectator will voice their dissent.
“That’s not your card?”
“What was it then?”
“The jack of spades.”
You sit in thought for a few breathless moments. Then your face lights up.
“Of course! The jack of spades. Yes. Sorry, this is a principle I still don’t totally grasp. Fortunately, I did actually manage to sneak your jack of spades under the ten of hearts…”
Point to your card box.
As we all know, the new Bicycle boxes have a ten of hearts on the front. You can probably see where this is going.
“Lift up the card box.”
Alternatively, “Open the card box.”
Laff in their faces. GTFO.