Experience
A ship engine failed and no one could fix it.
Then they brought in a chap with 40 yrs. on the job.
He inspected the engine very carefully, top to bottom.
After looking things over, the guy reached into his bag and pulled out a small hammer.
He gently tapped something. Instantly, the engine lurched into life.
The engine was fixed!
7 Days later the owners got his bill for 10k.
'What?!" the owners said
"You hardly did anything.
Send us an itemized bill."
The reply simply said - tapping with a hammer. $2
Knowing where to tap - $9,998
Don't Ever Underestimate Experience.