switcheroo

A switcheroo is a sudden unexpected variation or reversal often associated with a joke (sometimes "the old switcheroo"). It is often used colloquially to refer to an act of intentionally or unintentionally swapping two objects.

As a comedic device, this was a favorite of Woody Allen; for a time, he used so many switcheroos that friends referred to him as "Allen Woody." Some of Allen's switcheroo gags were:

• Carrying a sword on the street, in case of an attack it turned into a cane, so people would feel sorry for him
• Carrying a bullet in his breast pocket; he claimed someone once threw a Bible at him and the bullet saved his life.