OK, so I'm completely ripping this joke off from some guy's Craigslist post, but it's funny enough that it bears repeating.
A rope walks into a bar, takes a seat, and says to the bartender, "I'd like a beer please." The bartender, clearly angry, says, "No way. We don't serve ropes here." The rope, confused, walks out the front door. He stands there for a moment before, thinking there must've been some sort of misunderstanding, he walks back inside.
The rope sits down again, and the bartender immediately says, "What did I tell you? We don't serve your kind here!" He picks up the rope and tosses it out the window behind the bar.
Outside, the rope sees a bucket and decides to ask him what's going on. "Excuse me, bucket," says the rope. "Why does the bartender keep throwing me out? All I want is a beer." The bucket replies, "I don't know why he refuses to serve ropes, but I have a solution for you: disguise yourself and go back in. The bartender will never know."
"Thanks!" the rope says, pulling a few loose strands from his ends and tying them up before heading back inside. He sits at the bar once more and says, "One beer, please."
"Hey," says the bartender. "Aren't you that damn rope I keep kicking out?"
"Nope," says the rope. "Frayed knot."
(So it's no duck/hardware store joke, but it's close.)