Fly door on a door way. Summer is such a wonderful time of year – sunshine, holidays, longer days, everyone in a good mood. The only downside? Flies. Open any door or window and, within seconds, my house is full of them. They have no problem getting in. Getting back out, however, appears to be a mystery they've yet to solve. On the hunt for a natural solution, I came across these magnetic fly doors, which use velcro (and push pins if needed) to quickly attach securely to door and window frames.