The Gag:

Wager that you can change a $1 bill into a five. Insist that you can do it with any dollar bill under tight supervision.

This should be presented as a magic trick. Roll up your sleeves and spread your fingers before showing everyone the funny solution!

The Secret

Roll up the bill and bend it to look like the number five. Remember you said would change it into a five, not a five dollar bill!

If your audience is amused, then say, “Hey if you liked that, then you’ll love the one gramps showed me! Here, pull my finger!”