If you try hard enough by staring at the center and forcing your mind to accept what you know to be true then the square will return to its proper shape. The reason it looks to be distorted is because of the proximity to the straight lines that the circular ones make, which spread out farther as they approach the corners. It is only a ho-hum and predictable optical illusion.
Here is a
frighteningly real illusion. The lines are actually straight and keep their rigidity even while their contents are changing.