Glass cup shattered on its own, anyone have an explanation?
Best Answer - Chosen by Asker
If it was a heat resistant glass like Pyrex, I've heard stories that pieces manufactured years ago are more stable but newer pieces tend to shatter occasionally. Boron is added to reduce expansion which is why they're heat resistant, but these pieces tend to be thicker and if the factory used the low expansion as an excuse to cool the pieces more quickly, you'll have internal stresses similar to Corelle.
Now for the trigger. Glass accumulates scratches from normal use even if you can't see them. These scratches are weak spots, and if a scratch is at a point in the glass where there's a lot of residual stress it forms a traveling crack. The crack will follow the stress, literally unzipping that tightly bound spring that I mentioned earlier. The crack travels at several times the speed of sound and can spread outward, exploiting other weak points (scratches with stress underneath). We see it as unexpected explosive shattering.
That light tap was all it took to pull the trigger.
Plain glass also accumulates surface scratches and can break with only a light tap, but it's much less likely because they aren't manufactured with stress on purpose. A crack will follow weakness in the glass and you'll get a few solid pieces. But it can happen and this is the reason you shouldn't put your hand inside a glass to wash it. Use a brush.