I was wondering if you could answer a Brain Teaser:

A man is sitting with two women, seemingly identical twins. One of the
women always tells the truth, the other always lies, but the man does
not know which one is which. The women have served him a pair of
drinks, one of which contains a tasteless, odorless, but deadly
poison, the other the cure for a fatal disease he has contracted.
Before choosing which glass to drink, the man may ask one question of
one of the women. Can you formulate a question that would guarantee
the safety of the drink chosen?

If you could answer this question, that would be great! :)  Please
provide as much detail as you can as to how you got your answer.

Hello.

Here's a way to ask it...

"If I were to ask the OTHER twin which drink contains the poison,
which would she point to?"

The liar would point to the non-poisonous drink because the other twin
(the truthteller) would actually have pointed you to the poisonous
drink.

The truthteller would also point to the non-poisonous drink because
the other twin (the liar) would falsely claim that to be the poisonous
drink.
 
Either way, you've been pointed to the non-poisonous drink.



For a discussion, see:
Ask Dr Math - Liars & Truthtellers
http://mathforum.org/dr.math/faq/faq.liar.html


-------
search strategy
"one always lies" twins

I hope this helps.

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Question 41963
Submitted on 11/18/2003

A man is sitting with two women, seemingly identical twins. One of the
women always tells the truth, the other always lies, but the man does
not know which one is which. The women have served him a pair of
drinks, one of which contains a tasteless, odorless, but deadly
poison, the other the cure for a fatal disease he has contracted.
Before choosing which glass to drink, the man may ask one question of
one of the women. Can you formulate a question that would guarantee
the safety of the drink chosen?

Solution 41963

Assume both sisters know which drink is poisonous and which is the cure.
Randomly pick one of the two women. Ask her, "If I ask your sister to
pick the poisonous drink, which one will she pick?" Then drink the one
she points out.
If the woman being asked is the truthful one, then the other sister is
the liar. The lying woman would have picked the drink that cure since
he asked for the poison.
If the woman being asked is the liar, then she would lie about her
honest sister's pick, who would have honestly picked the poisonous
drink. Therefore, the liar would have pointed to the drink that cure
his disease.