They're sitting in the gutter outside and feeling really thirsty when one piece of string says "Hey! I've got an idea to get me into the bar."
So he starts twisting and turning, wriggling this way and that, pulling out a few threads here and there. His mate's looking at him and thinks he's gone completely nuts.
Then the piece of string walks back into the bar. The bartender looks at him a little suspiciously again and says "Here, you're not a bit of string, are you?"
The piece of string replies "No, I'm a frayed knot