One last optical illusion for now, and then I'll go back to something else. A Necker cube is a bistable image of a cube where it is unclear which is the front face and which is the back face. Here's a more complex version built of Modulex by Brixe63. Are the small cubes pointing forward from larger cubical recesses, or are the larger cubes pointing forward with little small-cube-shaped bites taken out? Does rotating the whole thing change that perception?