You charged me $100.00 and you only hit it with a Hammer!

A lady turned up at the workshop one day, She had a "Job" with her, she inquired if I could fix it, and fix it, while she waited. Usually we do not do that, but, from what she explained, it seemed as to be a relatively simple thing to figure out. So I took the job and put it on the bench, had a look at it, for a while, had a little think, unscrewed a panel, got a small hammer & drift, and gave the offending component a couple of good solid whacks, I tried the unit again, and lo and behold it was free and I had fixed it. With that I buttoned it up, made the Invoice out and presented both to the lady. She took a look at the account and spluttered, you...you.. charged my $50.00 and all you did was hit it with a hammer. my reply was polite and to the point, Madame, I said, I did NOT charge you $50.00, to merely "Hit it with a hammer", No, I said, I only charged you $1.00 to hit it with the hammer, BUT $49.00 to KNOW WHERE TO HIT IT, she promptly took out, a nice new shiny $50.00 bill, smiled and left. She has been a customer ever since.