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xkcd #3254: Detector
(thelemmy.club)
Title text:
No other experiment has a lower false negative rate.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3254/
If you can see the light has gone out, it just means the light bulb needs to be replaced.
So it's a "detector is working" detector