A duck walks in a bar.

“Got any crackers?”

An astonished bartender says “No” and the duck waddles away.

A week later the duck returns.

“Got any crackers?”

The bartender - less shocked - says “No” and the duck waddles away.

A week later, and a week after that, and a week after that the scene repeates itself, until one day the bartender yells “Look! We have no fucking crackers, and if you come in here again asking for crackers I am going to nail you bill to the bar!” So the duck waddles away.

A week later the duck returns and sees a livid bartender and asks “Do you have any nails?” The bartender, astonished once again replies “No.”

“In that case, have you got any crackers?”