They called the maintenence man when the washing machine broke down. He duly arrived, pulled the machine out from the wall, studied it for a few minutes then got a large hammer and gave it a mighty whack.

It started straight away.
He then handed the couple a bill for $50.
"Fifty bucks?" said the husband. "All you did was whack it with a hammer."
"It is all itemised on the bill," said the tradesman. "Hitting the machine with the hammer is $2. Knowing exactly where to hit it is $48."