If you watch the slide show here
http://www.linkslearning.org/Kids/1_Math/2_Illustrated_Lessons/4_Line_Symmetry/index.html, you'll see that some of the slides were made by unlocking the X/Y zoom and using masks. The first few are fairly simple, but then get more complex as the show progresses.
The challenge is to create a tutorial show with text instructions showing how to make a slide like one of the ones used in the show.
Anna J