Shamelessly lifted from reddit:
A physicist, mathematician and an accountant walk into a bar and the barman asks for the answer to 2+2.
The physicist creates a hypothesis and undergoes a series of experiments and screams that the answer is 4!
The mathematician went through a whole box of chalk to produce a 16 page proof report and yells that the answer is 4!
The accountant sneaks up to the barman and whispers “what do you want it to be?”
Xanthippa's Chamberpot 