Liar

You face 2 people. One always tells the truth, and 1 is a liar.

You can ask 1 question, the same to each.

How can you discover which one is the liar?

Originally posted by Duncan Clarke
You face 2 people. One always tells the truth, and 1 is a liar.

You can ask 1 question, the same to each.

How can you discover which one is the liar?

You don't need to ask the same to each.
One question suffices to any one of them.

The answer is you ask both of them this question - If I was to ask the other person which one will tell the truth, what would they say?
If you are asking the lier he will lie and say its him. If you ask the one that tells the truth he will know that the other one will always lie and so he will point to the other person. So that identifies the lier as both will have indicated the same person.

couldn't you simply ask the question, what day is it today?

http://www.giantitp.com/comics/oots0327.html

you asks them if they are celebrating their 200th birthday. The liar will say yes...

Originally posted by peka
you asks them if they are celebrating their 200th birthday. The liar will say yes...

But it won't tell you which way to go.

Because the puzzle is supposed to be at a fork in the road.

Originally posted by AThousandYoung
But it won't tell you which way to go.

Because the puzzle is supposed to be at a fork in the road.

does the lair HAVE to lie?

or is that a stupid question?

Originally posted by st dominics preview
does the lair HAVE to lie?

or is that a stupid question?

Yea

Originally posted by AThousandYoung
But it won't tell you which way to go.

Because the puzzle is supposed to be at a fork in the road.

Classic Johnny Carson

A fork in the road

YouTube

Go to 6:00 minutes unless you enjoy watching Carol Wayne.

Someone had to do this, so I did.

Point to one of them and ask the other "Would he call you a liar?" If the answer is no the one you are asking is the liar. If yes the one you are pointing at is the liar. Sorted with one question.

Originally posted by Julian Davies
Point to one of them and ask the other "Would he call you a liar?" If the answer is no the one you are asking is the liar. If yes the one you are pointing at is the liar. Sorted with one question.

Correct answer - ask Mr Left the following: "If I asked Mr Right which way to go, what would he say?"

Let us assume North is the correct direction.

If Left is the liar, then Right would say North, but Left would change the answer to South.

If Left is the truth teller, Right would say South, and Left would correctly report the answer as South.

Either way, you get the wrong answer, and go the other way.

It's even simpler than that. You don't even need the other one.

"Would you say Left is the correct way?"
Yes: Go left.
No: Go right.

Originally posted by Jirakon
It's even simpler than that. You don't even need the other one.

"Would you say Left is the correct way?"
Yes: Go left.
No: Go right.

I don't follow...

Originally posted by Duncan Clarke
You face 2 people. One always tells the truth, and 1 is a liar.

You can ask 1 question, the same to each.

How can you discover which one is the liar?

This is a bit of a trick question.

The question that you must ask is "are you a liar?"

The person who always tells the truth, MUST answer "no"

The person who is a liar MAY answer "yes". If he does, you have identified the liar. Otherwise, you can't.