They only have one I.
Instead of 'knock knock', they say, 'Arrr you there !'
A buck-an-ear.
I.C. !
Because they often spend years at C EDIT: made it more punny
Because they lost an I.
Two friends meet together and one asks: What would you do if you won the lottery? -I would build a brothel! Oh, and if it went wrong and you loose money? -I'd open it to the public