Follow the staircase around. Can you determine the lowest or highest step?
What happens when you go around in a clockwise direction?
What happens when you go around in a counter-clockwise direction?
So what's Happening?
The is a physical model of an impossible staircase designed by genetist Lionel Penrose. It is the first impossible object ever made and served as an inspiration for M. C. Escher's famous print that incorporates this staircase, "Ascending and Descending."
The actual model is separated at the right stair, but you can't see the split, because your visual system assumes that it is seeing this model from a non-accidental point of view; hence, it assumes that the stairs are joined.
Although the staircase is conceptually impossible, it does not interfere with your perception of it. In fact, the paradox is not even apparent to many people.
Although M.C. Escher and Lionel and Roger Penrose are the ones who made the impossible staircase famous, it was independently discovered and refined years before by Swedish artist Oscar Reutersvard. The Penroses and Escher, however, were unaware of Reutersvard's designs.
Since that time there have been innumerable adaptations and variations on the impossible staircase by not only Roger Penrose, but Oscar Reutersvard as well.
In the 1960's Stanford psychologist Roger Shepard made an auditory version of the impossible staircase.
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