I always enjoyed looking at Necker cubes and making them morph, as well as other optical illusions. I'm looking forward to trying the more "advanced techniques" mentioned here, like perceiving both morphs simultaneously or doing two cubes at a time. My intuition is that many of these exercises could be valuable training for the mind, although I definitely see how some (like the audio time compression thing or the 2d visual plane thing) could cause psychosis or other problems in a vulnerable individual. A few weeks ago I was wondering why optical illusions were part of my elementary school curriculum in the US. I doubt there is any connection at all to this particular Russian school, but I think "awareness and control of perception and attention" could be part of why you'd want a child to see them. Also, working with the "Schulte table" reminded me a lot of looking at passages or charts on standardized tests, or looking over logs or source code while debugging.