Technical Sketching Optical Illusions

Here are a couple of interesting optical illusions. When I went through college one of the courses I had to take was Technical Sketching. Many of my sketches looked similar to the below drawings but I think all of my drawings could actually be constructed.
I could be wrong, my professor gave me an "A" but if my drawings were even close to these he may have questioned my sanity and my mother always taught me that when dealing with a crazy person to just smile and back away slowly. :-)