I like these Lego Escher thingies:



The web site for them, complete with explanations as to how they did them, is here: http://www.andrewlipson.com/escher/ascending.html .

Here we have Escher--re-imagined:



Ow.

I haven't done one of these in a while.  These two are actually advertisements:





This is sometimes also called "environmental art."  Neat.

A delightful Photoshop:



My brain hurts!

This one's really bizarre!  Which image do you see first?



Did you see the old couple or the guitar player serenading the woman?

Are the lines wavy or straight?( Okay, I moved it behind a cut because it hurts the eyes . . .  )

Check with a straightedge!

Which sphere is bigger?




They're both the same size.

I love optical illusions. Here's today's.  Does this window open inward or outward?