Knowing

"EVER HEAR THE STORY of the giant ship engine that failed? The ship’s
owners tried one expert after another, but none of them could figure
out how to fix the engine. Then they brought in an old man who had
been fixing ships since he was a youngster. He carried a large bag of
tools with him, and when he arrived, he immediately went to work. He
inspected the engine very carefully, top to bottom. Two of the ship’s
owners were there, watching this man, hoping he would know what to do.
After looking things over, the old man reached into his bag and pulled
out a small hammer. He gently tapped something. Instantly, the engine
lurched into life. He carefully put his hammer away. The engine was
fixed!
A week later, the owners received a bill from the old man for ten
thousand pounds.
'What?!' the owners exclaimed. 'He hardly did anything!' So they wrote
the old man a note saying, “Please send us an itemized bill.”
The man sent a bill that read,
Tapping with a hammer...................................£2
Knowing where to tap................................£9998