There is a nice optical illusion showing the ambiguity of the way our brains interpret convexity and concavity. An image, especially of some sort of relief, can appear convex as well as concave. The brain usually automatically switches between interpretations when the image is turned upside down. Presenting both orientations at once makes the difference even clearer.
The ambigram above showcases this illusion. Where it reads 'convex' it looks like it pops up from the background, but when rotated it not only reads 'concave', it also seems as if it is impressed into the background. That makes two illusions in one image.
I got the idea for this ambigram from M.C. Escher's litograph Convex and Concave, which is based on the same illusion.