A duck goes into an ironmongers, hops up on the counter and asks: "Do you have any duck food?"
"No," says the ironmonger, "I don't sell duck food."
"Oh right," says the duck, and leaves.
The next day, the duck is back. He hops up on the counter again and asks: "Do you have any duck food?"
"No," says the ironmonger irritably. "I told you that yesterday, I don't sell duck food!"
"Oh right," says the duck, and leaves.
The next day, the duck comes back again. He hops up on the counter and asks: "Do you have any duck food?"
"NO!" Shouts the ironmonger. "I told you that yesterday, AND the day before! This is an ironmongers! I sell nails and screws and the like! NOT duck food! And if you come back again and ask for duck food I'll nail your feet to the counter!!!!"
Duck departs quickly.
The next day .... the duck is back. He hops up on the counter and looks at the ironmonger. "Have you got any nails?" he asks.
"No," says the ironmonger grumpily. "I'm out."
"Well then, have you got any duck food?"