Shape Tweens allow you to change the shape (morph) of an object over time.
It operates on objects drawn using tools such as the rectangle, oval, or pencil tools.
It does not work with grouped objects (such as text) or symbols. Those objects will need to be broken apart (Right-Click > Break Apart)
Draw a shape on your stage.
Create a new keyframe later on your timeline
If you want to modify your shape use the Selection (V) or Free Transform (Q) Tools.
Right-click in between these two keyframes and select Create Shape Tween.
To control more complex or improbable shape changes, you can use shape hints.
Shape hints identify points that correspond in starting and ending shapes.
Shape hints contain letters (a through z) for identifying which points correspond in the starting and ending shapes.
Shape hints are yellow in a starting keyframe, green in an ending keyframe, and red when not on a curve.
Select the first keyframe in a shape-tweened sequence.
Select Modify > Shape > Add Shape Hint. The beginning shape hint appears as a red circle with the letter a somewhere on the shape.
Move the shape hint to a point to mark.
Select the last keyframe in the tweening sequence. The ending shape hint appears as a green circle with the letter a somewhere on the shape.
Move the shape hint to the point in the ending shape that corresponds to the first point you marked.
To view how the shape hints change the shape tweening, play the animation again. To fine-tune the tweening, move the shape hints.
To add more shape hints, repeat this process. New hints appear with the letters that follow (b, c, and so on).
Select View > Show Shape Hints. The layer and keyframe that contain shape hints must be active for Show Shape Hints to be available.
Drag it off the stage.
Select Modify > Shape > Remove All Hints.