A giant ship's engine broke down and no one could repair it, so they hired a Mechanical Engineer with over 30 years of experience.

He inspected the engine very carefully, from top to bottom. After seeing everything, the engineer unloaded his bag and pulled out a small hammer.

He knocked something gently. Soon, the engine came to life again. The engine has been fixed!

A week later the engineer mentioned to the ship owner that the total cost of repairing the giant ship was $20,000.

"What?!" said the owner.
"You did almost nothing. Give us a detailed bill."
The answer is simple:
Tap with a hammer: $2
Know where to knock and how much to knock: $19,998