I put it in the other thread he posted the riddle in I'll get it
One Guard always lies, and one always tells the truth. That is absolute. So when Sarah asks Guard A to tell her what Guard B would say, she knows it will be a lie, no matter what. If Guard A were the liar, he would have to lie about what Guard B would have said. Since Guard B would have told the truth, Guard A would then have to tell the lie to Sarah. However, if Guard B were the liar, he would obviously have lied. However, Guard A would then have to answer the question truthfully which would be the lie from Guard B. Hence, no matter which guard is the liar and which guard is the truth teller, the answer that Sarah gets would have one and exactly one lie in the answer. Knowing this, Sarah takes the other road.