"Ducks love grapes" joke

A duck walks into a convenience store. He asks the man at the counter, "You got any grapes?"
Guy at the counter says, "No, we don't have any grapes."
Duck says "okay." and he leaves.
The next day the duck comes back in and says "You got any grapes?"
The man once again replies, "No! We do not have any grapes."
The duck says "Okay." and he leaves.
The third day the duck walks in again and asks, "You got any grapes?"
The man is very annoyed and says, "No! For the last time, we do NOT have any grapes. If you come in here again and ask for grapes, I'm gonna nail your bill to the floor!"
The duck replies "Okay," and leaves.
The fourth day the duck returns once again and asks, "You got any nails?"
The man at the counter says "No."
The duck says, "Well then, you got any grapes?"