# Find the liar

PinkFloyd
Science 11 Jul '08 23:27
1. 11 Jul '08 23:27
I saw this puzzler on a rerun of "Special Unit 2" last week (gosh I miss that show). There were 2 female mutants: 1 always lied, the other always told the truth. The police guy couldn't figure out which was which, but his partner asked a question and said "she's the liar". She then explained how she deduced this, but she talked so fast I didn't catch it. Does anybody know the answer, or did the writers just make this whole puzzle up?
2. 12 Jul '08 09:27
There was a puzzle like this at the puzzle section a while ago. Was about 2 brothers at a road crossing I think. But can't recall the answer.
3. flexmore
Quack Quack Quack !
12 Jul '08 10:16
Originally posted by PinkFloyd
I saw this puzzler on a rerun of "Special Unit 2" last week (gosh I miss that show). There were 2 female mutants: 1 always lied, the other always told the truth. The police guy couldn't figure out which was which, but his partner asked a question and said "she's the liar". She then explained how she deduced this, but she talked so fast I didn't catch it. Does anybody know the answer, or did the writers just make this whole puzzle up?
If you hold 1 finger up and ask the mutant how many fingers are being held up ... then the answer will make it obvious.
4. 12 Jul '08 16:111 edit
I have seen a similar puzzle to this.

You are in a dungon and there are two doors. By each door there is a knight. One knight always tells the truth, the other knight always lies but you do not know which one is which. One door leads to freedome, the other to certain death.

You are allowed to ask one question to one of the knights and then must choose which door to go through. What question must you ask to garentee choosing the door to freedome?

This version does not allow you to first ask an obvious question (like how many fingers?) to find out the one that is truthful.

Now for the answer, look away now if you dont want to see the spoiler before attempting it

You ask the following questions to either one of the knights:

"which door would the other knight tell me is the door to freedom?"

They knight will answer with a door and you then go out the other door.

This is because if you had happened to ask the lieing knight, he would know that the other knight is truthfull and that the other knight would tell you the door to freedom, as you asked. But he always lies so he will point out the door that leads to death. If you happened to ask the truthful knight he would know the other knight always lies and that the other knight would point to the death door. But he always tells the truth so he will point at the door to death. You therefore will always get the door to freedom is you go out the other door, the one that whichever knight you happened ask the question to does not indicate.

edit: to apply this to the original situation with the mutants, you would ask either one of the mutants: "which one of you would the mutant over there say is the lier?" You then know that the mutant that is choosen in response to the question is the one that tells the truth.
5. AThousandYoung
West Coast Rioter
12 Jul '08 21:121 edit
Originally posted by PinkFloyd
I saw this puzzler on a rerun of "Special Unit 2" last week (gosh I miss that show). There were 2 female mutants: 1 always lied, the other always told the truth. The police guy couldn't figure out which was which, but his partner asked a question and said "she's the liar". She then explained how she deduced this, but she talked so fast I didn't catch it. Does anybody know the answer, or did the writers just make this whole puzzle up?
"If I asked the OTHER alien if she was a liar, would she say yes?"

The liar knows the other one would say no, so she says yes, because she's a liar.

The truth-teller knows the other one would say no, so she says no, because she tells the truth.
6. 13 Jul '08 00:16
What if the liar saw through your plan and did a double bluff, to make sure the answer you got was a lie?
7. flexmore
Quack Quack Quack !
13 Jul '08 03:59
Originally posted by Tyrannosauruschex
What if the liar saw through your plan and did a double bluff, to make sure the answer you got was a lie?
That is very true ...
from wiki:
A lie (also called prevarication) is a type of deception in the form of an untruthful statement with the intention to deceive, often with the further intention to maintain a secret or reputation, protect someone's feelings or to avoid a punishment. To lie is to state something one believes is false with the intention that it be taken for the truth by someone else. A liar is a person who is lying, who has previously lied, or who tends by nature to lie repeatedly