There are lots of little clues/hints about this in the movie—many little things have a meaning—but we're not very "tight" with it, because we are hoping people will have their own ideas about the story, and make a new version of the movie. In this way (and others) we tie the story of the movie with the "open movie" idea."
The movie has many subtle clues and hints, each with a specific meaning, but we haven't been very detailed about them because we want viewers to come up with their own interpretations of the story and create new versions of it. This approach is also used in other ways to connect the film's narrative with the concept of an "open movie."