Our next illusion is the Cafe Wall, here in LEGO by Katie Walker. In 1973, Steve Simpson, a student working with psychologist Richard Gregory, was eating lunch and noticed that the black and white bricks in the wall appeared to follow curving lines, but when you looked close you could see they were straight. One explanation for this geometrical-optical illusion is based on physical differences in how the receptors in the retina perceive the black and white area. The thought is that the black areas bleed over and stimulate the nearby receptors that should be seeing the white area. And so instead of looking square, the black shapes seem to flare out a little as trapezoids. Since the mind sees that these all come together in a pattern, it tries to fit trapezoids together, which leads to the perception of lines sloping this way and that. But they're straight, really. If you don't believe me, hold a piece of paper up to your screen and you can see that every edge is parallel.