god i wish i could post the jimmy carr answer to the OP.
instead
Cheese sandwich walks into a bar. Barman says "sorry sir we don't serve food"
Bloke walks into a bar and says "have you got any helicopter crisps?" Barman says "sorry sir we only serve plane"
Blokes walks into a bar with a frog growing out of his head. "my god" says the barman "how did that happen?"
"well" says the frog "it started out as a boil on my arse"
did you hear about the constipated mathematician? he worked it out with a pencil