According to Canadian novelist W. O. Mitchell, this little-known adjunct to Newton's Third Law of Motion states that a privy will always fall with the door facing down. This was described in one of his short stories about of Crocus, Saskatchewan. The story involved the kids getting ahold of some dynamite, which they intended to use to make a hole for a fort. A dreadful miscalculation ensues... I think the story is in How I Spent My Summer Holidays.