A duck walked into my bar and asked, "Got any grapes?" Me - the bartender, confused, tells the duck that no, my bar doesn't serve grapes.
The duck thanks me and leaves.
The next day, the duck returns and says, "Got any grapes?" Again, I told him that, no, the bar does not serve grapes, has never served grapes, and, furthermore, will never serve grapes.
The duck, a little ruffled, thanked me and leaves.
The next day, the duck returns, but before he can say anything, I began to yell: ''Listen, duck! This is a bar! We do not serve grapes! If you ever 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, I say no.
''Good!'' says the duck. ''Got any grapes?''