Tracking down the origin of such human errors can be tricky since, from the exterior, the concrete looks fine. The supervising engineer of the building of JFK Airport noticed through routine tests that the concrete arriving on trucks before noon had good strength when it set, but that arriving just after noon was substantially weaker. Puzzled, he investigated all possible reasons for this but was unable to find the answer until he resorted to following the truck delivering the concrete on its journey to the airport. He found that around noon the driver was in the habit of taking a break for
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