A woman calls in a plumber when her washing machine breaks down. The plumber arrives, studies the machine, then produces a hammer and gives it a hefty whack.
The washing machine starts working again and the plumber presents a bill for £200.
'Two hundred pounds?' says the woman. 'All you did was hit it with the hammer.'
So the plumber gives her an itemised bill: 'Hitting washing machine with a hammer - £5. Knowing where to hit it - £195.'