Whoah! Never expected that Lima’s illusion would trigger so many reactions (over 400 in less than 24 hours)! Unfortunately, not that many of you guessed it right. It seems that even I was tricked by it. My guess was that the hand is in front of the object, with bars painted on it to make it appear as it was positioned behind the structure.
Anyhow, the real solution was kindly provided by Joao Lima: “The trick is very simple. The wall is very thin and on the other side there is the same structure, where the shadow is a projection behind the wall. I mean, the spot light is behind the wall so there is no reason to look for the hand shadow.
This illusory object was created by an artist called Joana Corona, and can be seen at the exposition at Oscar Niemeyer Museum in Curitiba – Brazil.” In the mean time, check this animated .gif file showing Escher’s Relativity in action! Whoever incorporated the escalator surely did a great job, and completely understands M.C. Escher and his work!