Imagine handing a spectator an envelope with a written prediction seen inside. Randomly chosen audience members freely name anything; favorite foods, destinations, celebrities, dates, or any personal information only they would know. Then, unbelievably, you reveal those exact details in your prediction!
OR, your envelope can clearly display a single playing card in full view. Then any card from any deck is signed and lost in the shuffle. Again, unbelievably, the card that is visibly removed from the envelope is the card just signed by the spectator!
To be clear...this is an envelope with an almost full front window that's designed to switch any object that's in full view under seemingly impossible conditions.