10. The Modern Mousetrap
Because the door is now a device that can instantly kill the mouse, the walls and the ceiling are no longer necessary for trapping it (in fact, they hinder it and become irrelevant). Once the walls and ceiling disappear (like two of the horses toes did), the door (now known as the hammer) is allowed to grow larger and stronger making it the efficient mouse killing device that we know today.
If the house has no use for mice that are trapped live, this hammer mousetrap will outdo the competition and the "humane live mousetrap" will become extinct, leaving us with a seemingly irreducibly complex mousetrap and freeing us up completely to do more important things around the house.
(Follow the "back" links to see how the "irreducible" mouse trap is "reducible".)