The teacher should point out that the tower's state can be changed only by manually invoking the method. Point out that the mouse could be outside the world, in which case the mouse information will be null. Remind students that the act() method is constantly running during the game and that it should check whether the object has been clicked and only then invoke the changeControl() method. Highlight that the logic for processing the control should be encapsulated inside a separate methodprocessUserControl().