A duck walks into a feed store and asks, "Got any duck feed?"
The clerk tells him, "No, we don't have a market for it it so we don't
carry it."  The duck says, "Okay" and leaves.

The next day, the duck walks in to the feed store and asks, "Got any duck
feed?"  Again the clerk says no and the duck leaves.

Next day, the duck walks in, and asks, "Got any duck feed?"  The clerk
says, "I've told you twice, we don't have duck feed, we've never had duck
feed and we never will have duck feed.  If you ask me again, I'll nail your
feet to the floor."  The duck leaves.

The next day, the duck walks in and asks, "Got any nails?"
"No."
"Got any duck feed?"