It's the sudden heat change that does that. Another thing not to do is take a hot dish out of the oven and put cold water in it. I don't actually put any water in them right away; I let them cool first.

I checked our cups. They are all bear the PYREX(R) branding, and say, "Made in the USA". They also say "Safe for microwave use", and "Not for stovetop use". I don't know if the bakeware says that, but I'd assume the same rule, unless otherwise stated. In the oven, the temp goes up only gradually.

waves