And I am at a complete loss as to why. I have three little boys so normally I might think one of them threw something....but I KNOW they didn't because I know what they were doing at the time. It s cold. VERY cold....12 degrees with a 3 degree windchill. My house has double pane insulated windows. The only thing I can possibly imagine caused it is this: My middle son is the most hot natured of our family. He had his bedroom window open just a fraction of an inch. The heat registers are on the ceiling and his is right in front of the window. Is is possible that the glass was so cold that when the heat kicked on it broke it? It is a straight crack, right across the middle on the inside glass. I am not too thrilled right now. Just kinda adds to the scrooge mood I have been feeling all this season.