The manager of a fancy hotel gets an urgent call from maintenance. "The pipes are going crazy, the basement is flooding!" Not missing a beat, he calls a highly recommended expert plumber.
The plumber shows up, and they both go downstairs. Sure enough, the pipes have burst, water is spraying everywhere, and it's already three feet deep. The plumber walks over to the central unit. He takes a minute to look around, then calmly takes out his hammer and gives it a single whack. Instantly the water stops shooting everywhere, and begins to drain. Everything is back to normal.
The manager thanks him profusely and asks for the bill. The plumber hands it to him. "$10,000."
"WHAT?" the manager says. "All you did was come down here whack the thing with a hammer. How can that be $10,000??"
The plumber responds, "Oh. I can give you an itemized bill if you want."
The manager agrees, and the plumber writes a new one. "Price to come down and whack the thing with a hammer: $100. Consulting fee—knowing exactly where to whack—$9,900."
Recently, I had a project on which I had to tag every change I made with a specific reason for the revision or suggestion. It was very eye-opening. Of course, we all know on some level that our knowledge and experience is informing our work and adds value, but the exercise of breaking that out really drove home just how many things an expert in any field begins to take for granted, but which are mysterious to most people.