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?
You could ask anybody, except the first man...
No matter what he is, he must have answered "I'm a knight" (either
truthfully, for he is one, or lying, if he isn't), so the second man is
a knight, and the third a liar.
You could either ask the second man, and do as he says, or ask the third man, and do the opposite.
There remains a question... must everybody in the island know whether
the bridge is safe or not? Or is it so obvious? And, in that case, why
should I have to ask anybody, instead of taking a look by
myself? ;-)
|
Posted by e.g.
on 2004-03-26 11:14:32 |