Logic puzzles train the same muscles as proof-based mathematics — laying out what you know, what you can deduce, and what's still unknown.
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Knights always tell the truth, knaves always lie. You meet two: A says 'we are both knaves'. What's A?
If A is a knight, then 'both knaves' is true — but a knight isn't a knave. Contradiction. So A is a knave; the statement is false; so they're NOT both knaves. So B is a knight.