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