On the island, what do knights and knaves do?

Knights always tell the truth, knaves always lie

Knights lie, knaves tell the truth

Why can no islander ever say I am a knave?

A knight would be lying and a knave would be telling the truth

One guard lies, one tells the truth. Which question finds the safe door?

What would the other guard say is the safe door? Then pick the other door

Knights and Knaves Quiz: Truth-Teller and Liar Logic

Raymond Smullyan made islands of truth-tellers and liars famous in his 1978 book What Is the Name of This Book?, and this knights and knaves quiz gives you 10 of his style of puzzles. Play online, read each statement carefully and decide who is a knight, who is a knave and which question opens the right door.

Start Knights and Knaves Quiz

🎯 Daily Trivia Challenge • 🧠 10 Questions • ⏱️ 150 Seconds

Question 1 / 10 Time: 150s

Rules
On the island, what do knights and knaves do?
Impossible
Why can no islander ever say I am a knave?
Both
A says: We are both knaves. What are A and B?
At least
A says: At least one of us is a knave. What are A and B?
Same type
A says: B is a knave. B says: A and I are the same type. What are they?
Mirror
A says: B is a knight. B says: We are opposite types. What are they?
If
A says: If I am a knight, then B is a knight. What are they?
Ask
What does every islander answer to the question Are you a knight?
Doors
One guard lies, one tells the truth. Which question finds the safe door?
Author
Which logician popularized knights and knaves puzzles?

Explanations: answer key for all 10 questions

Q1: On the island, what do knights and knaves do?
Answer: Knights always tell the truth, knaves always lie
Every puzzle rests on these two rules, so test each possibility against them.
Q2: Why can no islander ever say I am a knave?
Answer: A knight would be lying and a knave would be telling the truth
A knight cannot say something false, and a knave cannot say something true, so the sentence is impossible.
Q3: A says: We are both knaves. What are A and B?
Answer: A is a knave, B is a knight
If A were a knight, the statement would be false, so A is a knave; since the statement is false, B must be a knight.
Q4: A says: At least one of us is a knave. What are A and B?
Answer: A is a knight, B is a knave
A knave saying this would be telling the truth, so A is a knight, and the true statement makes B the knave.
Q5: A says: B is a knave. B says: A and I are the same type. What are they?
Answer: A is a knight, B is a knave
If A is a knight, B is a knave whose claim of sameness is false, which fits; the reverse leads to a contradiction.
Q6: A says: B is a knight. B says: We are opposite types. What are they?
Answer: Both are knaves
If A were a knight, B would be a knight telling a lie, which is impossible; so A is a knave and so is B.
Q7: A says: If I am a knight, then B is a knight. What are they?
Answer: Both are knights
If A were a knave, the if-statement would be automatically true, which a knave cannot say; so A is a knight, the statement is true and B is a knight.
Q8: What does every islander answer to the question Are you a knight?
Answer: Yes
Knights say yes truthfully and knaves say yes as a lie, so the question reveals nothing.
Q9: One guard lies, one tells the truth. Which question finds the safe door?
Answer: What would the other guard say is the safe door? Then pick the other door
Both guards point to the wrong door when asked this, so you choose the opposite.
Q10: Which logician popularized knights and knaves puzzles?
Answer: Raymond Smullyan
Smullyan's puzzle books made truth-teller and liar problems a classic of recreational logic.

Knights And Knaves Quiz – How to Solve Truth-Teller Puzzles

Knights and knaves puzzles take place on an imaginary island where knights always tell the truth and knaves always lie. Raymond Smullyan popularized them in his 1978 book What Is the Name of This Book?

The simplest method is to assume a speaker is a knight, follow the logic and check for contradictions, then try the opposite assumption. Statements such as I am a knave are impossible, and the question Are you a knight? always gets the answer yes.

The classic two-door riddle, where one guard lies and the other tells the truth, is solved by asking what the other guard would say and choosing the opposite door. Later puzzles add normals who can do either.

What This Quiz Teaches

Puzzle fans and logic students will enjoy these questions.

What the island rules are
How to test assumptions
Which statements are impossible
How to solve two-door riddles
Who made the puzzles famous

Did You Know?

The two-door riddle appears in the 1986 film Labyrinth.
Smullyan was also a magician and pianist.
These puzzles are used to teach Boolean logic.
Some variants add spies who may lie or tell the truth.
Truth tables help solve harder versions.