There is a theory that for a puzzle game to interesting it has to be NP-hard. Something about how otherwise your brain is too good a latching onto the "trick" and the game is boring.
They are difficult. As I'm not a native English speaker, I didn't know many of the obscure words or usages, so I actually played these games from a purely computational perspective. I discovered early on that there were a lot of at least NPC problems in them. As my English improved (partially thanks to these games), intuitions began to help me take shortcuts, as if I had become a nondeterministic Turing machine.
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