A duck walks into a bar and asks, "Got any grapes?"
The bartender, confused, tells the duck no. The duck thanks him and leaves.
The next day, the duck returns and asks, "Got any grapes?"
Again, the bartender tells him, "No -- the bar does not serve grapes, has never served grapes and, furthermore, will never serve grapes." The duck thanks him and leaves.
The next day, the duck returns, but before he can say anything, the bartender yells, "Listen, duck! This is a bar! We do not serve grapes! If you ask for grapes again, I will nail your stupid 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?"