The bartender says no, and the duck leaves. The next day, the duck returns and asks, "Do you have any grapes?" The bartender again says no, and the duck leaves.
The day after that, the duck walks in the bar again and asks the bartender "Do you have any grapes?"
The bartender, losing his patience, screams at the duck, "I told you duck, I don't have any grapes and if you ask me again I will nail your feet to the floor!!"
The duck looked startled and leaves.
Two days later the duck returns walks up to the bar and asks the bartender, "Do you have any nails?"
The bartender replied, "No," and the duck said, "Good! Got any grapes?