On an island of truth-tellers and liars, one islander says: 'I am a liar.' What is he?
You're a stranger on an island where every inhabitant is either a knight, who always tells the truth, or a knave, who always lies. One islander walks up and says, 'I am a knave.' Can you work out whether he's a knight or a knave?
Reveal the answer
Neither is possible — the statement traps both types in a contradiction. A knight would never falsely call himself a knave, since that would be a lie. A knave can never truthfully call himself a knave, since that would be the truth. No consistent islander could actually say this; it's a self-referential paradox in disguise, a simpler cousin of the Liar Paradox. Logician Raymond Smullyan built an entire genre of puzzles on this knight-and-knave premise in his 1978 book.