The Adelson checkerboard shadow optical illusion.
Which mouse is standing on the darker square – answer, neither!
The checker shadow illusion is associated with the work of Edward H. Adelson
This is how the illusion works.
An object (the elephant) on a checkerboard casts a shadow on the board.
The parts of the board in the shadow are darker than the parts of the board that aren’t.
The human brain knows that the squares on the board are all same – either white or dark gray, so it compensates for the darkening effect of the shadow, making the white squares that are in the shadow seem lighter than they really are (which can be as dark as the dark gray squares, as here).
The second image shows the mice holding a uniformly gray strip across the checkerboard to allow the tones of the squares to be compared