I would be interested to make a short video to show how we can use geometry to explain some things related to an optical illusion. The "spinning dancer" is a rotating silhouette, which appear at the second sight that you can't tell in which direction rotates. At first sight it seems that everybody choses a particular direction.
The script is something like this
- introduce the illusion
- let the viewer decide in which direction she thinks the ballerina rotates
- explain why two people out of three see the dancer spinning clockwise
- explain why the correct answer is that the dancer spins counter-clockwise
(I explained the last two points here, using simple geometry in space and some elementary notions on perspective)