A few days ago, my son and I were in the car and decided to pass some time by coming up with a story together. There was a guy. A pirate. And he had a dog named Lucy. And then there was this other pirate. And so on.
When we got home, I thought I'd show him what a "storyboard" was (it's basically a series of - usually sketchily drawn - images that shows you every frame of a commercial/tv show/movie and how it'll be shot). Once we had the storyboard, it made sense to start putting together costumes. And once we had the costumes, well...
We obviously had to make a movie.