A piece of string
No serious joke (and there's yet another oxymoron) site about the English language could be complete without a collection of puns. Here's one of them.
A piece of string is walking down the street. (Yes, a piece of string). It's tired and thirsty, so starts looking for a pub. Eventually, he finds one, but just above the door he sees a sign: "String. No Service."
So he keeps going. He turns down a different street and finds another pub. Same thing. "String. No Service."
In fact every pub he finds has this prohibitive sign above the door, and the string decides it's time for desperate measures. So he pulls apart his threads, ties himself into a big knot, and rolls into the pub. "A pint of your finest ale!" he demands.
The publican leans over to him and asks "Are you a piece of string?" to which the string replies "No, I'm a frayed knot."