Shattered

I remember twice in my life pouring hot water into a glass and the glass shattered. I learned from this and now when I make tea in a glass I know to squint and tense up as I pour from the kettle; in other words I didn't really learn much of anything except to expect it to happen again. (Frisson: the feeling I get when except and expect appear near to each other in a sentence--what is that wordplay called, semianagram proximity rhyming? Maybe it's a dyslexic thing! Am I dyslexic?)

So far a glass hasn't shattered again. But what are the variables? When will a glass shatter while receiving hot water?

My usual sources for this kind of knowledge, the Straight Dope Archives and Guardian Notes & Queries have been useless on this.

Physics books say that unequal expansion of different temperature zones aggravates the glass and causes a stress fracture. And I know lab ware doesn't crack often. So I assumed thinness helps. But the vessels I use right now are thick: two tall tumblers and a short glass mug, all thick. Thickness should cause shattering.

I have occasionally encountered an old wives tale that a metal spoon should sit in the glass as a prophylactic during pouring. Based on specific heat and conductivity this makes no sense and I discard it as wrong and even completely backwards, a Kentucky hickism on the order of saying very-close veins for varicose veins.

How to get to the bottom of shattering hot glass? Can anyone offer an anecdote of pouring hot water into a glass and seeing it crack and what kind of glass it was or other experimental details?