Visual illusions are fun, but they are also insightful (Carbon, 2014)—the great pedagogic value behind such illusions is that most readers, while being amused, also experience perceptual insights which assist the understanding of rather complex perceptual processing (see Gregory, 2009). Here, I present a very simple illusion which was inspired by a discussion which I involuntarily witnessed in a German photographic shop back in the year 1989, when photoprints in Germany were typically made in sizes of 9 × 13 cm (small purchase option) or 13 × 18 cm (large option). The customer who was currently being served was complaining about the incomprehensible billing policy of the shop—from his perspective, the large purchase option was only marginally larger than the small option, but incomparably more expensive. Indeed, when we looked at the specific setting, with the smaller sized print being centered on the larger one, the larger photo print looked only marginally larger. Ignoring all mathematically based means, the vendor followed this perceptual path, puzzling about the pricing policy himself. After a while, I chimed into the conversation with a simple mathematical argument: to compare the size of a 9 × 13 cm and a 13 × 18 cm print, just “cancel the common 13” and you will get a doubled remaining side as 18 = 2 × 9, so the larger option is 100% larger than the smaller one, the size is doubled! Both attendees looked through me, fully confused, a bit shocked and very doubtful . . . doubled? No way! They did not find basic mathematical rules convincing at all. So I decided to change my persuasion strategy (purely for practical reasons; I was late and wanted to be served quickly!) by visually demonstrating the fractional arithmetic via a very subtle, but extremely insightful change of the configuration: simply by rotating the smaller print by 90° and positioning it in such a way that the large edge was aligned with the small edge of the large print, both attendees exclaimed “wow!” in unison. Instantly, they had understood the size relationship of the two prints—the larger print was really double the small one; exactly double!