Shape Tweening morphs one shape into another. You have learned by now that Motion Tweening must be done on instances of symbols. Shape Tweening, on the other hand, must be done on shapes.
To create a Shape Tween, place a shape in a keyframe. Add a blank keyframe later on the same layer with a different shape in that keyframe. Hoover over the proper frame and layer, then Right-click on the timeline between the two keyframes and select Shape Tween.
Open our example, saved as AdvancedAnimation/demos/ShapeTween.fla.