Penrose Stairs

The particular optical illusion featured in today's gag is called the "Penrose stairs" named after its creator Lionel Stairs. The Penrose stairs were later made famous by the graphic artist M. C. Escher (whose art style I have attempted to ape in today's comic).

Having knocked on hundreds, if not thousands, of doors as a missionary, I'm pretty sure I tracted a couple of these as a missionary.