7. Separation
Since the nook has evolved to have a floor, a ceiling, three walls and a door, it can become a separate system independent of the room. Separating itself from the room allows us to place the new "trap" anywhere the mice may be. This greatly increases it's ability to catch mice. It has become an ancestor to the "humane live mousetrap".
Note: Separation of the nook could have happened at any time.