Wednesday, March 17, 2010
One day the warden was bringing a prisoner to be executed, but was feeling sorry for him. The prisoner had been convicted on dubious charges and DNA evidence had been unearthed that might have exonerated him, but the court threw it out. So the warden wanted to give the prisoner one more chance at freedom. There were two guards with them and the warden whispered in their ears, out of earshot of the prisoner. The warden instructed one to tell nothing but lies when asked a yes-or-no question and the other to tell only the truth when asked a yes-or-no question. Otherwise they were to remain silent. The prisoner couldn't tell which was the liar and which was the truth-teller.
The warden then told the prisoner that he could ask one yes-or-no question to one of the guards. If, from the answer, he could pick the correct door to freedom, the guards would look the other way.
What question should the prisoner ask?
Point to a door and say to one guard, "If I asked the other guard if this were the door to freedom, would he say yes?" The answer will be a lie no matter whom you ask. Even the truth-teller, when speaking on behalf of the liar, would be forced to give a false answer. So if the answer is yes, choose the other door. If the answer is no, choose the door you pointed to.
Posted by Charlie Guthrie at 7:01 AM