Two pieces of string walk into a bar

Two pieces of string walk into a bar. One of them sits down while the other goes up to the bar and says, "two pints of Guinness, please." The barman looks the piece of string up and down and says, "Sorry, but we don't serve pieces of string here."

So the piece of string goes back to his friend and explains the situation, and the friend decides to tie himself into a knot and fray his ends. He walks up to the bar and orders two Guinness's. This time the barman starts to fill the order but halfway through he looks up and says, "Hold on a minute... You're not a piece of string are you?" To which the piece of string answers:

"No, I'm a frayed knot."