A critical gearbox failed and no one could fix it. So they brought in a bloke with great trouble-shooting experience. He inspected the gear drive very carefully. After looking things over, the guy reached into his bag and pulled out a small tool.
He gently tapped something. Before long, the gearbox was back working! 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. .$50 Knowing where to tap. .$9950 Don't underestimate experience.