Ask people if they’ve seen illusions on TV. Particularly that Zig-zag trick where the magician pulls the girls middle away from her body. It’s that same trick… but close up.
Tell them you don’t have any big boxes, but you want to show them that trick anyway.
You thought you could probably use their bill (Yes, you can use ANY BORROWED BILL) and a playing card from your pocket… you’re a magician, you always have playing cards on you.
The secret is in the precision-made card gimmick. 100% self-contained.
It’s incredibly durable and constructed to withstand a level 9000 skeptic.