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?

The first guy, if he was a liar, would have said he was a knight.

If he was a truth teller, he would have said he was a knight.

So we know that the first guy is always going to say he is a knight.

The second guy said that the first guy said he was a knight, which is always true.

Therefore, it is safe to say the second guy always tells the truth, so it is safe to ask him if the bridge is safe or not. 

Posted by Philip on 2005-04-04 19:21:55