Lee WOne islander, one sentence, and it tells you exactly what the other one is.
On an island, knights always tell the truth and knaves always lie, and everyone is one or the other. You meet two islanders, Alice and Bob. Alice says, “We are both knaves.” What is Bob?
Start with Alice. Could she be telling the truth?
If Alice were a knight, her words would be true and she’d be a knave, which can’t happen. So she’s a knave and what she said is false. They aren’t both knaves, so Bob is a knight.
The usual wrong answer is knave, from taking a known liar at her word.
Source: After Raymond Smullyan, What Is the Name of This Book?, 1978