Well, thermal expansion will cause cracks in things, but I've never heard of a windshield doing it on its own. My theory, without knowing too much, is there was already a hairline crack that became worse after being subjected to the warmer temperatures (thus expanding the glass). Now remember, though, that it takes a great deal of energy to heat a windshield...it's a big dual-layered piece of glass, otherwise known as an insulator. One way to check to see if there's been a significant temp difference is to put your hand on the windshield. My guess is it'll still be pretty cold, especially if you've been driving and not just sitting there (though I wouldn't recommend playing while driving...that would just be plain irresponsible )