Optical illusions in pencil

I was being drawn to do some geometric art. Back when I was young, I loved geometry...not so much the theorems and postulates, but how lines worked together to create forms of all sorts. While researching ideas, I came across a page of optical illusions, so I have recreated them here.
The first one is a blivet (who knew there was an actual name for it?) or "the devil's fork". The other drawing is a Penrose triangle or "impossible triangle". Both are very reminiscent of M. C. Escher, one of my favorite artists. I may continue with some more...maybe some tesellations?