The clerk says no, and the duck leaves.

10 minutes later the duck comes back into the store and asks, "Do you have any strawberries?"

The clerk, a little agitated, says "NO!" and the duck leaves.

10 minutes later the duck returns. "Do you have any strawberries?"

"For the 3rd time, NO!!! If you come back and ask again I'll nail your webbed feet to the floor!" The duck leaves.

30 minutes later the duck comes back into the store. "Do you have any nails?" "No" says the clerk. The duck asks, "Do you have any strawberries?"