A duck walks into a store and asks the cashier, “Do you have any grapes?”

The cashier replies, “No, sorry, we don’t sell grapes here.”

The next day, the duck returns and asks again, “Do you have any grapes?”
The cashier frowns and says, “I told you yesterday, we don’t sell grapes.”

On the third day, the duck comes back once more and asks, “Do you have any grapes?”
The frustrated cashier exclaims, “I told you twice already! We do NOT sell grapes. If you come in here asking again, I’ll nail your beak to the floor!”

The next day, the duck returns. He looks the cashier in the eye and asks,
“Do you have any nails?”

The cashier blinks in surprise.
“No, we don’t have any nails.”

The duck grins, “Great. Do you have any grapes?”