…or is it? Have fun wracking your brain on this one. It’s an illusion invented, my sources tell me, by Lionel and Roger Penrose, mathematician, physicist, and also well-known among skeptics as a proponent of the idea of quantum neurobiology.
It was depicted by M. C. Escher in 1961, but earlier conceived by Swedish artist Oscar Reutersvärd, though neither Escher nor either Penrose are thought to have known it.
This is a good illustration of how our own senses can fool us, and a useful perceptual example of why direct experience is not as consistently reliable as pseudoscience advocates would have us believe.
(Last Update 2011/1/11, 4:30, Crediting Corrections)