This one will perhaps only be good for your more advanced students.

[coryzinho]

This one will perhaps only be good for your more advanced students.
A piece of string walks into a bar and asks for a drink. The barman refuses to serve him saying rudely, 'Sorry but we don't serve the likes of you. Get out!'

The piece of string leaves the bar feeling glum, he walks down the road and then he sees two girls who he asks for help. 'Please,' he says to one of the girls, would you tie a knot in me?' This she does. 'Please,' the piece of string says to the other girl,'would you mind taking your comb and fluffing out the ends of my string?' so the girl obliges.

'Thank you' says the string and he turns around, goes back into the bar and immediately orders a drink again.

The barman looks at him quizzically and says 'aren't you the piece of string that was in here a moment ago?'

'No' came the answer, 'I'm a frayed knot.'