Escher's trickery revealed!
Friend and fellow "blogger" Ned Gulley found Andrew Lipson's LEGO™ interpretations of Escher's prints, including "Ascending and Descending." In trying to copy the print in three dimensions, Lipson found that the picture was of a spiral and not a circuit, and that the circuity was only apparent from a specific point of view. I guess the real trick is that Escher didn't build the model before carving his woodblock, so he had to mentally envision the spiral and the correct vantage to give the illusion of infinite stairs. Certainly more clever than all that fish becoming birds business. Here are some Escher quotes from Jon's "Daily Escher" calendar:

"We adore chaos because we love to produce order."

"I have sometimes heard painters say that they work 'for themselves,' but I think they would soon have painted their fill if they lived on a desert island."

"I'm starting to speak a language which is understood by very few people."

"The other day I gave a lecture to a Rotary Club in Hilversum... The audience included three medics who had made their diagnosis of my 'case' by the end of the lecture - obsessional neurosis. I suggested to my publisher that he use this as a title, but he said no."