Experience and the mallet

Have you heard this one? A machine breaks, the assembly line is idle, and nobody can fix it. Finally they call the old guy who retired and he agrees to come fix it. When he arrived, he listened to the machine for a moment, and then hit the machine with a mallet. Machine fixed; assembly line working again.

He sent the client a bill for $5000. The bill was even itemized:

hitting the machine with a mallet - $50.00
knowing where to hit it -- $4950.00

Sometimes you need an old guy (like me) who knows how things really work.