A New Illusion for Pisa Tourists

How many thousands of tourists have shot the same exact picture as the one you see here? I couldn't even imagaine the number. I'll just say every single person who ever snapped a picture of the leaning tower of Pisa has done this. I may be wrong, but I think what they're doing is actually playing with forced perspective. One of my favorite science websites, Cognitive Daily, now offers another totally cool illusion you can create. Go here to see it. Then will somebody explain to me how it works?-

How many thousands of tourists have shot the same exact picture as the one you see here? I couldn't even imagaine the number. I'll just say every single person who ever snapped a picture of the leaning tower of Pisa has done this. I may be wrong, but I think what they're doing is actually playing with forced perspective. One of my favorite science websites, Cognitive Daily, now offers another totally cool illusion you can create. Go here to see it. Then will somebody explain to me how it works?
--David Schonauer