Reminds me of an old engineer's story
From the papermaking industry - huge machines a quarter mile long, running at 70mph or more, hot, noisy, and very very expensive if they're not running all the time.
The machine stops, and nobody from the company can work out what's wrong. The old fella who used to be their chief mechanical engineer has just retired, so they call and beg him to come in "as a consultant". They promise to pay him pretty much anything he can justify, as long as he comes in RIGHT NOW. So he comes in.
Wanders up and down the machine for an hour, tapping this, twisting that. Eventually he takes a bit of chalk out of his pocket and draws an X on a part, saying "Replace that", and pops off home. The part is replaced, and lo, the machine runs like a Swiss watch.
The old fella's invoice comes in... for fifty thousand quid. A letter goes back "We appreciate you saved us more than this fixing it so quickly, but can you justify this? i.e. can you itemise the invoice?"
Letter comes back labelled "Itemised bill:
1. Chalk Mark - one pound.
2. Knowing where to put it - forty nine thousand....