Lost in the woods, you finally happen upon a rickety old bridge across a deep ravine. The ravine is too steep to go down and climb back up. You need to know if the bridge is safe. As 'luck' would have it, you recognize that on the other side of the bridge is that dreadful town, "Knight-and-Liarville". Everyone who lives there is a knight (who always tells the truth) or a liar (who always lies). You are tired and you've been lost in Knight-and-Liarville before. You see three men on the other side of the bridge.

You shout out: "Are you a knight?"

The first man says something, but you can't hear what he said.

The second man shouts, "He said he was a knight."

The third man shouts, "No, he didn't. He said he was a liar."

Which man do you ask to find out whether or not the bridge is safe?

Since there are no knaves here, the first person could not have said they were a liar.  If a liar says he's a liar, he's telling the truth and so we have a contradiction.  A knight can't say he is a liar either.  So the first person must have said he was a knight, though we have no way of knowing if that is the truth or not.

Next, since the second and third person made opposing statements about the same thing (what the first person said), one of them MUST be a liar and one of them MUST be a knight.

Since we already determined that the first person said he was a knight, that means that the second person is a knight.

So you have to ask the second person about the safety of the bridge.

Later!

Posted by nikki on 2004-03-26 09:43:44