Kris,
Great math activity. Have you ever considered this as a cross curricular activity with art? I tried doing tessellations with my 5th graders and it failed miserably. I think it is one of those activities that you need more abstract thought processes to understand. Of course I am not a math person so maybe it was my inability to teach. I wish I had more access to computers so students could at least have the chance to explore the web sites you told us about today. I have a very large Escher print that you may borrow anytime. It is like the one with the waterfall but I think it is a better example of the optical illusion. It could really make a bold statement to get your kids involved.