The order was for two thousand jars of apricot jam, and the workshop had never made more than three hundred.
On paper the arithmetic looked finished before it began. The old recipe made fifty jars from twelve kilos of fruit, so two thousand jars would want forty times as much — four hundred and eighty kilos. The owner wrote the number down, and then did not order the fruit.
Instead she made fifty jars again, and weighed everything.
Twelve kilos went into the pan. What came out filled only forty-four jars. Some fruit stayed on the stones, some clung to the sides, and a good deal left as steam. To fill a true fifty she needed fourteen kilos, not twelve. Scaled up, the order needed five hundred and sixty kilos — eighty more than the paper had promised, and enough to have stopped the line on the second day.
"The recipe is not wrong," she told the two women who work the pans. "It is just not a measurement. Nobody ever weighed it."
There is a habit hidden in that morning, and it is worth more than the eighty kilos. Before trusting a rule, run one concrete case through it and see what comes out the other end. A single real example costs an hour. A rule that has never met a real example can cost a season.
The jam went out on time. The note she pinned above the stove is still there: weigh the small batch first.