A piece of string goes into a bar and says to the bartender, "I'll have a beer, thanks."

The bartender replies, "Sorry, sir, but we don't serve string here."

So the string goes outside. The string stops a passerby and asks, "Here, help me get these ends crossed over themselves, and rub me against that brick wall over there." The passerby complies and goes on his way.

The piece of string heads back into the bar and says to the bartender, "I'll have a beer, thanks." Puzzled, the bartender says, "Wait a minute - Aren't you that piece of string that was just in here?" The string replies, "No, I'm a frayed knot."