An island is inhabited solely by Knights (who always tell the truth) and Knaves (who always lie). You meet two residents, X and Y. X says: "At least one of us is a Knave." What are X and Y?
X is a Knight and Y is a Knave.
If X were a Knave, his statement would be true, which creates a contradiction because Knaves cannot tell the truth. Therefore, X must be a Knight telling the truth. For the statement "At least one of us is a Knave" to be true when X is a Knight, Y must be a Knave.
About this riddle
Logic riddles give you a few facts and ask what must be true. List what you know, cross out what cannot be, and see what is left.
Difficulty: Medium. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
More logic riddles
A detective interviews four brothers—A, B, C, and D—about a missing ring. Exactly one brother stole it. - A says: 'B took it.' - B says: 'D took it.' - C says: 'I did not take it.' - D says: 'B is lying.' If three brothers are telling the truth and only one is lying, who stole the ring?
Five merchants sit in a row at a circular table. Each merchant trades only in a single unique commodity: Silk, Iron, Spice, Glass, or Wood. 1. The merchant trading Silk sits directly across from the merchant trading Iron. 2. The merchant trading Wood sits immediately to the left of the merchant trading Silk. 3. The merchant trading Spice does not sit next to the merchant trading Iron. Who sits immediately to the right of the merchant trading Spice?
Five runners finish a school marathon, and only these facts survive. Luca finished ahead of Felix. Opal finished ahead of Jonas. Jonas finished ahead of Nico. Felix finished ahead of Opal. Who finished last?
Five distinct houses are painted different colors: red, blue, green, yellow, and white. The green house is to the immediate left of the white house. The red house is somewhere to the right of the green house. The blue house is at one of the two ends. If the yellow house is not next to the blue house, what color is the house in the exact middle?
Four knights participate in a jousting tournament resulting in distinct ranks from 1st to 4th. Sir Kay did not finish last. Sir Lancelot finished immediately ahead of Sir Gawain. Sir Bors finished 2nd. Who finished in 1st place?
A warden offers three prisoners a chance at freedom. He shows them 3 red hats and 2 white hats. He blindfolds them, puts a hat on each, and hides the remaining two. Prisoner 1 (who sees P2 and P3) says 'I don't know my hat color.' Prisoner 2 (who sees P3) says 'I don't know my hat color.' Prisoner 3 (who is blindfolded and sees nobody) says 'I know my hat color!' What color is Prisoner 3's hat?