Straight or Not?

I found this at Archimedes-Lab. The graphic poses the question "Straight or not?" Being an optical illusion it wouldn't be posing the question if the answer wasn't that each is a square with perfectly straight sides.

Hard to believe? I agree so don't trust me get out the straight edge and check it out. Or if you must print it out and fold or cut along the edge. No matter how you check the image the edges will be straight every time.