You are standing in front of ten people, one of which is a truth-teller while the other nine are togglers . (A toggler is a person who toggles between the-truth and the-lie for every question asked.) You have no clue who the truth-teller is and who the togglers are.

You are given the chance to ask one and only one question to any, but only one of the ten people. From the provided answer you must be able to deduce with certainty the identity of the truth teller.

Question: So what is that question you will be asking?

  • 2
    $\begingroup$ presumably the people all know who are togglers and who is the truth teller, and you know that? $\endgroup$
    – Ewan
    May 1, 2015 at 10:41
  • $\begingroup$ Do togglers know in which state their fellow togglers are? $\endgroup$
    – Ivo
    May 1, 2015 at 13:00
  • $\begingroup$ @Ewan, was your question ever answered somewhere (and maybe subsequently deleted)? It's pretty important to the answer. $\endgroup$ Oct 7, 2015 at 0:43

2 Answers 2


"Which people here are truth-tellers"?

If the person you asked is a truth teller, he will indicate himself.

If he is a toggler in "truth" mode, he will indicate the truth teller.

If he is a toggler in "lie" mode, he will indicate the 9 togglers (this is my interpretation of telling "the-lie", and note they only toggle after each question and this is a single question). But as you know there is only one truth teller you know it must be the unindicated person.

  • 1
    $\begingroup$ Very nice. I guess in any case, you can be sure that the truth teller is the person that is classified like no other, similar to your last sentence. $\endgroup$
    – blakeoft
    May 1, 2015 at 12:07
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    $\begingroup$ Strictly speaking, they could lie by pointing out five people out of the 10. $\endgroup$
    – Glen O
    May 1, 2015 at 15:01
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    $\begingroup$ lie is often defined as something that is not true. so both partial or impartial truth is a lie and your method fails, becuase there is no surity that he may point a correct person or a wrong person. $\endgroup$
    – RE60K
    May 1, 2015 at 16:50
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    $\begingroup$ @IanF1 What if he says no-one? Seems like a fully blown lie to me. $\endgroup$ Oct 6, 2015 at 16:06
  • 1
    $\begingroup$ A sentence that is false is a lie. That "partial truth" does not make sense. $\endgroup$
    – miracle173
    Oct 6, 2015 at 17:30

"If I asked you another question immediately after this, and then I pointed to another person in your group and asked 'Are they the truth-teller?', who could I point to in order for you to say 'yes'?"

Makes use of the fact that the togglers always alternate between truth and lie.

  • $\begingroup$ If he starts in lying mode then his answer to the third question would be any one person that is not the truth teller, he is going to lie about that statement and name any other person which could still leave you with a false answer. $\endgroup$
    – Poelie
    May 1, 2015 at 9:54
  • $\begingroup$ @Poelie - I suppose that is a possibility. It's easily fixed, though. I'll update. $\endgroup$
    – Glen O
    May 1, 2015 at 9:59
  • $\begingroup$ I highly doubt he would understand your qustion :D.(the person) Please explain in detail (I can't get it sorry) $\endgroup$
    – RE60K
    May 1, 2015 at 17:00

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