Two pieces of paper the size of a $20 note are each placed in an envelope. A borrowed $20 note is placed in a third envelope after its number is first noted.
The owner of the $20 note is told to pick which envelope he thinks his note is in, but before doing so is told that you have so much confidence in his ability that you will burn th other two envelopes.
This is done and it is discovered that the audience member only has a piece of paper in his envelope. The $20 is discovered to have been in the possession of the president (or some VIP) all the time