Grapes

A duck walks into a store and asks the clerk, “Do you have any grapes?” The clerk says no.
Five minutes later the duck comes back and asks, “Do you have any grapes?” and again the clerk says no.
Five minutes later the duck comes back and asks, “Do you have any grapes?”
The clerk says, “No, and if you ask again, I’ll nail your feet to the floor.”
The duck comes back five minutes later and asks, “Do you have any nails?” The clerk says no. The duck says, “Good. Do you have any grapes?”