So now youve got 13 frames of an animated movie clip that...well...isnt really animated. To take care of that, lets go back to frame 1. Click on that frame in your timeline, and then use your Arrow Tool to select all of the shapes/fills of the fireworks burst on your stage. (Make sure youre on frame 1 when you do this.) Then use the Free Transform tool to scale the fireworks in the first frame down as small as you can, to the point of near-invisiblity.
Now comes the easy partthe part that we already covered in Lesson 2. All you have to do now is select all of frames 1-12, and then go to the Properties panel and select Tween -> Shape from the dropdown. Frames 1-12 should turn pale green, and an arrow will connect the keyframes in the first and last frames. When you scrub your timeline you should be able to watch the fireworks start from nothing and then grow larger.