All kids LOVE optical illusions, and this one is an easy one. I have seen this lesson posted on lots of different sites, so I really have no idea who originally came up with it. It really is a great one though.
Basically, students draw lines to a point in the middle of the page. Then, students draw slightly curved black stripes on each triangle. Lastly, students shade the triangles. The darkest color should be on the edges of the triangle, and the lightest in the middle. This makes it look more like a cone than a triangle. My students seemed to like this lesson very much.