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Winning Tic Tac Toe is my favourite:

Yeah, because computers can't win Tic Tac Toe at all, so that's a good way to separate computers from humans.

100% success rate for bots. How is that a useful CAPTCHA?

Well, the really smart ones know not to play.

"General, you are listening to a machine. Do the world a favor and don't act like one." -- Dr. Stephen Falken, "WarGames" (1983)