So, do y'all know why it breaks? Me neither, but I've got a pretty good idea...
-When something becomes hot the molecules expand, thus it wouldn't be unreasonable to think that all the glass molecules had expanded more than usual (ie, room temperature)
-By adding cold liquid, in essence the molecules slowed down\contracted. Really, though, what happened was the energy from the molecules in the glass transferred into the colder fluid, since cold is lack of energy.
-But pouring the cold liquid into the cup caused uneven distributions of cold throughout the glass. Some parts contracted faster than others. This means that some parts of the glass were pushing out while others were pushing in.
-The force generated by that was probably enough to force slightly warm areas away from slightly cool areas, cracking the material and breaking the glass.
There. I do know *some* physics, even if I haven't taken a class on one.