Open the door and there is a door. Open that door and there is another door. Open that door and there is a wall. You have gone nowhere. Behind the wall there is another wall. Behind that wall is another wall. And then behind that other wall are a billion more walls. Below that turtle is a turtle. Below that turtle is another turtle. Below that other turtle are more turtles than there are electrons in the universe. And then there are more turtles.