Two identical twins- One always lies, other always tell the truth.
What is that one question that you should ask to one of them to find out who is who?

Edited January 13, 20178 yr by The Almighty

If you were your brother (sister?) would you always tell me the truth?

If you were your brother (sister?) would you always tell me the truth?

Yasss!!

OK-- I have to ask. Let's postulate that "A" is the one that lies, and B is the one who tells the truth. If I ask "A" "If you were your brother (sister) would you always tell the truth?", the correct answer would be that B tells the truth, so "A" will lie and say say "No." If I ask B "if you were your brother (sister) would you tell the truth?", the correct answer would be "No", and that is the answer B would give, because B always tells the truth. Since both will answer "No", how can I meet the original challenge of telling which is which??

What if you ask them something you already know the answer to?

What if you ask them something you already know the answer to?

They might say "I don't know".

Even if you ask a question like "Do I have an elephant on my head?" they might still- for the purposes of a logic puzzle- say that they don't know.

Logic puzzles don't work if you apply common sense.

Since both will answer "No", how can I meet the original challenge of telling which is which??

 

 

Exactly.

 

I think the usual version of this problem is to find out which door to go through or something (one leads to treasure and the other to death). You ask the twins which door to take and then do the opposite.

I think it only works if you ask one twin what the other twin will say, and then do the opposite.

So to answer the original question, you ask one twin which one the the other would say was the liar. And then it isn't that one.

Edited January 15, 20178 yr by Strange

What if you ask them something you already know the answer to?

why don't you ask them : Do you have brother ?

the liar will answer NO !

Sorry but Fresh is completely correct (as was Daecon a few posts ago) - the question in the OP is poorly formed. One always lies the other always tells the truth - if vague prevarication is allowed then nothing will suffice - but if (as is normally assumed) you will get straight answers then any factual question (to which the answer is known)will get you a solution. It is only recursive / reflexive questions that suffer from a problem.

 

This problem normally requires you to find a piece of hidden knowledge - which door has a tiger behind - in addition to the problem of the liar and the truth-teller. The trick is normally to form the question in such a way that both brothers will answer identically - but when no other information is needed then the trick is to form a question that they will answer differently but predictably; any simple pre-known factual question will do