It's a weird thing. Here's a short story that's relative. I used to work in a shopping mall. A tee shirt store had these thick glass doors. No metal frame work. The door handle and the hinges were bolted thru the glass. These doors got a lot of use by customers, a lot. One day, a lady pushing a mall stroller, which was a metal frame stoller, hit the door edge. Just lightly, as she was turning in the doorway. The whole door shattered! No one was hurt, but the only thing still up were the hinges. The moral? If you hit something "just right" it'll break.