At least once a semester, I put up a big display board with lots of exampls and we incorporate elements of optical illusions into one project. This year, I decided to base the design on an op art heart/sphere we did last year. This time, we used 11X11 paper (so we could add a little 12X12 frame around it) and we used oil pastels instead of chalk. They did not blend as well as I wanted them to, but it was still a pretty successful project.
We focused on selecting an ornament shape that was symmetrical. I taught them how to practice on 6X6 newsprint, until they got a shape they were happy with. Then they could choose between green or red 11X11 paper. IN order to make the shape symmetrical, if they would draw it with oil pastel, they could simply fold the paper down the middle, rub the back of the paper, and the other half would lightly transfer to the opposite side, making it easy for them to draw the other half exactly like the first half. Then they could decorate the ornament how ever they wanted. I reminded them to draw an even number of 'rays' around the edge.