Truth and Lie Puzzles— Concepts, Formulas & Shortcuts
- Assume a person is truthful, propagate the consequences, and look for a contradiction.
- A truth-teller can never assert a false statement, and a liar can never assert a true one.
- A liar cannot say "I am a liar" — that would make the statement true.
- "At least one of us is a liar" can only be said truthfully, so the speaker must be a truth-teller.
- For the two-guards problem, ask what the OTHER guard would say and take the opposite.
- Check every surviving case against every statement before committing to an answer.
Truth and Lie Puzzles Practice Questions with Answers
Attempt each question first, then open the explanation. All 6 questions below are free to read and require no signup.
Q1.On an island, knights always tell the truth and knaves always lie. A says, "We are both knaves," referring to himself and B. What are A and B?
Difficult- ABoth knaves
- BA is a knave and B is a knight
- CA is a knight and B is a knave
- DBoth knights
Truth and Lie Puzzles question 1 of 6+Show Answer & Explanation
Answer: B. A is a knave and B is a knight
Explanation: A knight could not truthfully say this, so A is a knave and the statement is false. Since A is already a knave, the falsehood must lie in B — so B is a knight.
Q2.A says "B is lying", B says "C is lying", and C says "A and B are both lying". Who is telling the truth?
Difficult- AOnly A
- BOnly B
- COnly C
- DA and C
Truth and Lie Puzzles question 2 of 6+Show Answer & Explanation
Answer: B. Only B
Explanation: If C were truthful, A would be lying, making B truthful — contradicting C. So C lies, which means at least one of A and B is truthful. Assuming A truthful forces C truthful, a contradiction. Hence only B tells the truth.
Q3.You face two doors and two guards, one who always lies and one who always tells the truth, but you do not know which. What single question identifies the safe door?
Moderate- A"Are you the truthful guard?"
- B"Which door is safe?"
- C"What would the other guard say is the safe door?"
- D"Is this door safe?"
Truth and Lie Puzzles question 3 of 6+Show Answer & Explanation
Answer: C. "What would the other guard say is the safe door?"
Explanation: Both guards give the same wrong answer to that question — the truth-teller reports the liar's lie and the liar misreports the truth — so you take the opposite door.
Q4.A says, "At least one of us — A or B — is a knave." If knights always tell the truth and knaves always lie, then:
Difficult- ABoth are knaves
- BA is a knight and B is a knave
- CA is a knave and B is a knight
- DBoth are knights
Truth and Lie Puzzles question 4 of 6+Show Answer & Explanation
Answer: B. A is a knight and B is a knave
Explanation: If A were a knave the statement would be true, which a knave cannot say. So A is a knight and the statement is true — meaning B is the knave.
Q5.X says, "Y and I are of different types." Y says, "X and I are of the same type." Then:
Difficult- AX is a knight, Y is a knave
- BX is a knave, Y is a knight
- CBoth are knights
- DBoth are knaves
Truth and Lie Puzzles question 5 of 6+Show Answer & Explanation
Answer: A. X is a knight, Y is a knave
Explanation: If X is a knight, Y is a knave, and Y's claim of sameness is false — consistent. If X were a knave, his claim would be false so both would be knaves, but then Y's "same type" claim would be true, which a knave cannot make.
Q6.In a group where every member either always lies or always tells the truth, someone says "I am a liar." This tells you that:
Moderate- AThe speaker is a liar
- BThe speaker is truthful
- CThe situation is impossible
- DThe speaker is sometimes truthful
Truth and Lie Puzzles question 6 of 6+Show Answer & Explanation
Answer: C. The situation is impossible
Explanation: A truth-teller cannot call himself a liar, and a liar saying it would be telling the truth. No consistent assignment exists, so the scenario cannot occur.
Truth and Lie Puzzles — Frequently Asked Questions
What is the standard two-guards question?+
Ask either guard: "If I asked the other guard which door leads to freedom, what would he say?" Both possible guards produce the same wrong answer, so you take the opposite door.
Why can't a liar say "I am a liar"?+
Because the statement would then be true, and a liar never says anything true. Statements of that form immediately identify the speaker as a truth-teller or reveal the scenario as impossible.
Cleared the aptitude round? Now grab the role.
Preparation only pays off when you apply. Thousands of internships and fresher jobs across India are live on MyInternships.in right now — free to apply, no consultancy fees.