OK, 3 pieces of string go in to a bar, the first piece of string say they will get the drinks in.  They go to the bar and say 'Good barman may I have 3 pints of beer?', the barman says 'Are you a piece of string?', the first piece of string replies 'well yes I am', the barman says, 'I'm sorry but we don't serve pieces of string in here'! 

So the first piece of string goes back to the table and tells the other strings what happened. 

The 2nd piece of string says 'don't worry I'll get served' and with that he puts on a trilby, an over coat and some sunglasses, and then approaches the bar.  '3 pints of beer please' has asks the barman, the barman replies 'are you a piece of string in disguise?, because we don't serve pieces of string in here'.   

The 2nd piece of string goes back to the table and tells them what happened. 

The 3rd piece of string says 'I'll get the beers in just watch'.  He then proceeds to tie himself up in to unimaginable shapes and then pulls all of his ends in to bits.  He approaches the bar and asks '3 pints of beer please', the barman asks 'are you a piece of string?', the 3rd peice of string replies 'No, I'm a frayed knot'.