A duck that steals can be called one of two things: a safe quacker or a robber ducky. What does the duck say to the banker? "My bill is bigger than yours."
These duck jokes, like many other animal jokes, are created using word play. Most of these forms of jokes are considered puns, as they exploit multiple meanings of words that have the same spelling or similar sound of other words. Things like quack substituted for crack, fowl for foul, a duck's bill for a sales receipt, and the animal's name used as part of the reflexive act of ducking, are all common duck-related puns.