Heat Is On
The Bill Tube of The New Millennium
You borrow a note from a spectator and have them sign their name across the front. The signed note is then placed into an envelpe and "THEY" burn it. You joke about not being able to fix the bill because THEY burnt it themselves, so you offer to let them keep the lighter. You take the lighter back, open it up, and inside the lighter is the spectator's signed note!
The spectator had the lighter in their hand the entire time!