This a fun filler activity for times in math class when students finish their assigned work (or test) early and need something to do. While it takes about 15 minutes to fully show the students the method, once they know it they can entertain themselves for quite some time!
Students will need to know how to measure in millimeters to do this activity. Younger students might need more guidance.
I suggest draw the first layer or two with your students (on the overhead) so they can see the method clearly. Then, enjoy! Even the non-artist can create spectacular optical illusions with this method!