Using an idea I found here, I had the kids make optical illusions using only their hands, a pencil, and a Sharpie.
Students were instructed to trace their hand, very faintly, in pencil. Then, they would draw horizontal lines across the paper, raising the line slightly when they crossed over the hand, creating an arc.
Some kids started from the bottom, others went from the middle. I personally think the middle gives better results - I'm not entirely sure why, but it just seems to.
We noticed that the more lines you put, the more obvious the illusion becomes. If students take the time to do this slowly, it really shows the illusion!
A few students even acted as test subjects, and tried in different colors. Although any color works, I still love a nice, sharp, black the best, and I would encourage future classes to stick with black.
The one above looks like the hand is floating away in the wind. Love that effect!