A man was suffering a persistent problem with his house. The floor squeaked. No matter what he tried, nothing worked. Finally he called a carpenter who friends said was a true craftsman.
The craftsman walked into the room and heard the squeak. He set down his tool box, pulled out a hammer and nail, and pounded the nail into the floor with three quick blows.
The squeak was gone forever. The carpenter pulled out an invoice slip, on which he wrote the total of $45. Above that were two line items:
Hammering, $2
Knowing where to hammer $43.