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They can be logical [e.g. honestans/swindlecants or hats on death row], math [e.g. probability, sequences], physics or geometry, especially where intuition might suggest wrong answer.
Inductive, intuitive, but still requiring some thought (eg. riddle of the sphynx, What-Am-I riddles, anagrams, Shakeepuddn's style, etc.)
Post amazing illusions you have seen on the web right here.
Anything else on the web that is worth mentioning.