Half moon on an outhouse

My wife and I were discussing, after watching the movie "Shrek" why there is always a half-moon on an outhouse. Any ideas?

Two ideas. It's a crescent moon. Moons (of any phase) may not have universally adorned real outhouses when outhouses were common. They're a convention of cartoons and other forms of fiction, much like all businesses being named Acme. I hope someone else can say how the moons got started.