A penguin walks into a bar, and asks the bartender if he has any plums. The bartender, confused, tells the penguin that no, his bar doesn't serve plums. The penguin thanks him and leaves. The next day, the penguin returns, and again repeats his request for plums. Again, the bartender tells him that, no, the bar does not serve plums, has never served plums, and, furthermore, will never serve plums.
The penguin, a little ruffled, thanks him and leaves. The next day, the penguin returns, but before he can say anything, the bartender begins to yell: "Listen, penguin! This is a bar! We do not serve plums! If you ever ask for plums again, I will nail your stupid penguin beak to the bar!"
The penguin is silent for a moment, and then asks, ''Do you have any nails?''
Confused, the bartenders says "No."
"Good!" says the penguin. "Then do you have any plums?"