A man was suffering a persistent problem with his house. The floor squeaked. No matter what he tried or who he hired, nothing worked. Finally, he called a carpenter who friends said was a true craftsmen. The craftsmen walked into the room, and heard the squeak. He set down his toolbox, pulled out a hammer and a single nail, and pounded the nail into the floor with three blows. The squeak was gone forever. The carpenter pulled out an invoice, on which he wrote the total of $50. Above the total were two line items:
Hammering , $2
Knowing where to hammer, $48