A man pulls into a garage, his engine is making a horrible noise. The mechanic steps out, listens for a moment, and asks the man to pop the hood. He reaches into his pocket, pulls out a hammer and gives it a good WHACK.

The engine starts to purr beautifully.

"That will be $100"

"$100 for a smack with a hammer?!"

"No, sorry for any confusion. It is $1 for the smack with the hammer, it is $99 for knowing where to hit it."