Why A Duck?

A duck goes into a vegetable produce store and says, "Do you have any grapes?"

The man says "No."

The duck comes back in an hour and says, "Do you have any grapes?"

The man says "No, I told you I didn't have any grapes."

The duck comes back in another hour and says, "Do you have any grapes?"

The man says "No I told you I didn't have any grapes and if you ask me again, I'll nail your tail to the floor."

The duck comes back in another hour and says, "Do you have any nails?"

The man says "No."

The duck says, "Do you have any grapes?"