A duck walks into a hardware store and asks the clerk, "Got any grapes?"
The clerk replies, "This is a hardware store, we don't sell grapes."  The
duck leaves.

The next day, the exact same thing happens, and the duck leaves the
hardware store.

The third day, the duck walks in and asks for grapes and the store
clerk, slightly concerned, replies, "Listen, this is a hardware store, 
we sell tools and wood and things like that, we do not sell grapes, now,
if you come in here again and ask for grapes I will nail your feet to 
the floor!"  The duck leaves, very scared.

The very next day the duck enters the store and walks straight up to the
clerk.  The duck asks, "Got any nails?"  "All out," replies the clerk.
"In that case, got any grapes?"