In the café wall illusion, the horizontal lines are actually parallel, that is, the biochemical sensation of light hitting the eyes is horizontal; however, the perception is that they are not horizontal—after interpretation of the sensation by the brain, the lines do not appear parallel anymore. We have been trained over time to expect things (like wooden planks) to bend down when a block (like a brick) is placed on top of it. Therefore, the lines with blocks on top do not appear parallel. Hundreds of such visual illusions exist.