If the needle on an industrial sewing machine becomes badly worn, the article being sewn can be ruined. In traditional apparel factories, the people who operate the sewing machines monitor the needles and replace those that begin to wear out. Industrial sewing operations are becoming increasingly automated, however, and it would be is expected to become standard equipment in the automated apparel factories of the future.
What this question is testing
Anticipate
Why would a factory need a needle-monitoring device at all? If needles wore out at predictable rates — say, every 100 hours — you could just replace them on a schedule and skip monitoring entirely.
Evidence
The fact that monitoring (whether by people or by an acoustic device) is treated as necessary implies that needles wear unpredictably. You can't schedule replacements for something that wears at random rates.
Goal
Find the answer that captures this — needles wear out at unpredictable rates.
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