This is as simple as it gets
Create an object
Create a new layer and draw a shape (no stroke) that covers your object
The masked area will be revealed when we're done
(Yes, this is the opposite of how masks usually work)
Insert a keyframe (F6) down the timeline
Select the first keyframe and the Free Transform Tool
Resize the "not yet a mask" shape so that your original object is revealed and insert a Shape Tween
Remember, in Macromedia's universe this means that the object will be hidden in frame 1 because it's not yet covered by the mask (go figure)
Insert a frame (F5) down the timeline so that the object remains on the stage for the show
Position your cursor over the icon (or the "not yet a mask" layer's name) and...
Control-click and select Mask from the contextual menu
And a whole bunch of things happen (I'll explain this in class)
If you don't want your animation to loop endlessly, make a new layer, insert a keyframe, and stop action
Now, use your imagination and create something worth watching
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