Waterfall, by Mauritz Cornelis Escher.
Look at this exquisite Escher lithograph. The draughtmanship is nigh
perfect, or is it? Everything seems to be in place, yet we seem to be
in a strange universe where water flows uphill. What is wrong with it?
Well, nothing is wrong with it. It is just our minds trying to equate
the lines Escher drew with what out senses perceive of the real world.
The Escher's lithograph is an illusion, that you will readily agree. But
then all paintings, photographs, and drawings, are illusions, neither
more nor less than Escher's. Escher's Waterfall is based on the Impossible
Triangle of his friend and admirer Roger Penrose, the British mathematician
and physicist. In fact, Escher incorporated not one but two Penrose tribars,
as they are now known, in his lithograph.