Plate glass can actually be quite tough stuff.
Has anyone walked into a clean plate glass window or door that they couldn't see? It's like walking into a solid brick wall. I know, I've done it. It doesn't break, but it bloody hurts though.
Years ago, I had this old early 1960s B&W TV set. Eventually it packed-up, so broke it up for parts, as I was into electronics at the time. These old TVs always had thick plate glass in the front of the screen, because those old CRTs where just so dangerous with no implosion protection. I tried breaking the plate glass screen with a sledge hammer, it would not go. Ended up making a glass-top coffee table out of it in the end, so strong you could stand and jump on it quite safely.
When they smash plate glass windows and doors in the movies, it's NOT real.
Double glazing can be appear to be extremely tough as well, unless one knows the correct way to break it, that is impacting it at the corners.
Edited by MikeDT - 8/12/11 at 8:08pm