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There are 2 twins who think the same, look the same, sound the same, and act the same. The only difference is that 1 of them is male, 1 is female. Without asking the twins for their gender, how can you figure this out? You have a maximum of 2 questions.

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closed as too broad by klm123, Ben Aaronson, kaine, Aza Oct 7 '14 at 21:25

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ Does that also exclude implied gender questions like "Can you get pregnant?" $\endgroup$ – Kyle Hale Oct 5 '14 at 16:10
  • $\begingroup$ Unless by “act the same” you mean that they’ll give the same answer to every question, the top two answers both seem fine. If that is what you mean, well… “can I have a blood sample”? $\endgroup$ – Ry- Oct 6 '14 at 6:53
  • $\begingroup$ Does the term "twin" imply that there are no other siblings who where born at the same time? $\endgroup$ – Cephalopod Oct 6 '14 at 9:55
  • $\begingroup$ You can do it in zero questions - undress them. $\endgroup$ – Trenin Oct 6 '14 at 17:15
  • $\begingroup$ Do they always lie or always tell the truth? Or do they randomly lie? $\endgroup$ – Trenin Oct 6 '14 at 17:16
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If I asked your twin which gender you are, what would they tell me?

Even if they're both liars you'll get the right answer - as the one you're asking would lie about what the other would say, who would also lie.


For the people confused about this, in the world of riddles the person you're asking is an oracle who is generally omniscient (they know the answers to your questions), consistent (for a given question, they'll always answer the same way), and gives meaningful information.

It's also a common trope that when you have essentially a binary question to ask one of multiple oracles, they might be liars. Sometimes this is spelled out, like in the Knights and Knaves puzzle many are familiar with in the door riddle from the movie Labyrinth. Sometimes, as in this question, spelling it out would basically give away the answer.

A big clue here was that they "act the same" - which is otherwise out of place.

If the oracle is inconsistent (or doesn't know the answers in the first place) you can't get useful information at all - and you don't have a riddle.


What if they answer with gibberish?

Then you can't get meaningful information and don't have a riddle.

What if they answer inconsistently, lying only sometimes?

Then you can't get meaningful information and don't have a riddle.

What if they just answer "female" to every question?

Then you can't get meaningful information and don't have a riddle.

...

and so on.


So probably the biggest clue here is that it is in fact a riddle therefore you can throw the useless absurdities right out.

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    $\begingroup$ The puzzle doesn’t say anything about lying. Even if they can lie and will act exactly the same, they may still give different answers to the questions of “which gender are you?” and your solution. $\endgroup$ – Ry- Oct 6 '14 at 7:05
  • $\begingroup$ Finally someone who realized they might lie! +1 for only 1 question and not asking "Is the sky blue?" XD $\endgroup$ – warspyking Oct 6 '14 at 18:08
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    $\begingroup$ But what if they sometimes lie and sometimes tell the truth (that is to say, normal people)? This answer won't work then. $\endgroup$ – Moyli Oct 6 '14 at 19:22
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    $\begingroup$ They could just answer questions randomly for any reason. As you did not specify rules for truth tellers vs. liars there is no reason for this answer to make sense. If the boy, for instance, pretends to be a girl and the girl goes along with it, this answer does not work. $\endgroup$ – kaine Oct 6 '14 at 20:07
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    $\begingroup$ For all we know, they could just both answer "foobar" to any question, every time!!! $\endgroup$ – Falco Oct 13 '14 at 9:42
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Ask one: "Do you have a twin brother?" If answer is "yes", so this twin is female, otherwise this twin is male.

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    $\begingroup$ I agree - well done :-) $\endgroup$ – Tony Suffolk 66 Oct 5 '14 at 18:27
  • $\begingroup$ You're still missing something. $\endgroup$ – warspyking Oct 5 '14 at 18:54
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    $\begingroup$ @warspyking What am I missing? The question "do you have a (twin) brother?" seems to solve the problem... $\endgroup$ – nicael Oct 5 '14 at 19:02
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    $\begingroup$ There's a specific reason you require 2 questions. They act alike. Remember that. $\endgroup$ – warspyking Oct 5 '14 at 19:21
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    $\begingroup$ @warspyking In what situation could this not give the correct answer? $\endgroup$ – Kevin Oct 6 '14 at 3:03

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