There are impossible things!!!

[photo: lithograph Waterfall by Maurits C. Escher (1961). His waterfall recycles its water after driving the water wheel. If it could work, this would be the ultimate perpetual motion machine that also delivers power! If we look closely, we see that Mr. Escher has deceived us, and any attempt to build this structure using solid masonry bricks would fail.]

The fact that there are impossible things is hardly surprising. It's obvious (?) one can't draw a straight line with a compass. We already saw some less trivial examples. For example, I can't create a slider device that would allow switching between all slider puzzles available at this site. Such a device would constitute a mega-puzzle to which a switch will have to be provided, which in turn would create another mega-mega-puzzle and so on ad infinitum. Selfreferences allow for questions that could not be logically answered.