The filling line at a small water plant outside Chirchiq runs for eleven hours a day, and every bottle that leaves it is supposed to hold five hundred millilitres.
No machine is that exact. So the quality control rule is not written as a single number. It is written as a distance: a bottle is rejected if its volume differs from five hundred millilitres by more than eight millilitres.
Read carefully, that one sentence names two numbers that are never printed on the paper. Eight millilitres below the target value is four hundred and ninety-two. Eight above is five hundred and eight. Everything between those two limits goes into the box; everything outside them goes back to be filled again.
On Tuesday the inspector pulled three bottles off the line. The first held five hundred and five millilitres, which is five away from the target, so it passed. The second held five hundred and eleven, which is eleven away, and it went back. The third held four hundred and ninety-three — seven away, on the low side, and still allowed.
The shift supervisor keeps one note taped to the machine, and it is the whole idea of the rule in eight words: Not which side. How far.