The Breathing Square Illusion is an illusion that tricks your mind into thinking the rotating square is increasing & decreasing in size. However, this is simply just a trick, as when it passes behind the teal blocks, it appears to change size. However, upon remove the corner blocks, you can observe how the square is not, in fact, changing in size at all.
Note: The Orange Square is slightly off-centered due to canvas rotation functionality, but still provides a good reference.