Walking around car shows and cruise nights, it was always puzzling to me why so many of the cars have one or more cracked windows. Nice cars, too, not just old rusty beaters. And it seems like it's always the back ones. Now it's happened to me. I noticed today that my left rear window in my '37 has a crack. It wasn't there last week. I've rolled it down and up a few times (original regulators), and driven a few miles, but nothing I'd expect to crack a window. Why?