Cleaning the windows of a 737-800

When we were waiting at the terminal for flying back from Maui on Tuesday I looked out the window of the terminal building.Sitting on the window, all Dukes of Hazard style, was the co-pilot (based on position I'm guessing) hanging out of the plane with a bottle of water and a squeegee.I know how planes work. I know that this is literally almost a 50-year-old design for a plane and the kinks have long been worked out. But it's still a bit unnerving to see that the windows open like that. Like, what is preventing the window from opening when you're, say, flying over the Pacific.Just saying.