Both of the following links show this project which we tried in grade 4.

The use of pencil is great for teaching and re-enforcing tonal work.  My grade 4’s found it tough going but the majority understood and enjoyed the project.  It can take 2 hours of art unless they are a little slower in which case allow for a wee bit more time.

I let the kids see other optical illusions on the net and we talked extensively about what an optical illusion is and why we enjoy them.  They certainly loved the ones that appear to be moving!