Glass is funny that way. What looks like perfectly normal glass can have imperfections in it that will break at random times.
I used to work in a medical blood testing lab. We handled enough specimens that we routinely saw glass tubes shatter while nobody was touching them, simply from the change in temperature from shipping to room temperature where we were working. We really didn't like it when clinics would send us samples in glass tubes.
I also once saw a mercury thermometer snap in half upon warming, with nobody touching it and right when I was looking at it to check the temperature.
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