Suppose there are twin brothers, one which always tells the truth and one which always lies. (So in this case they both know what is true and false, or as you put it, both are accurate in their knowledge.) What one yes-no question could you ask to either one of the brothers to figure out which one he is?

Why this works

This riddle relies on the clever use of logic and the nature of truth-telling and lying. When you ask the first question, "If I were to ask your brother whether you always tell the truth, what would he say?", you are effectively creating a scenario where the truth-teller will say "no" because the liar would say he doesn’t tell the truth. Conversely, the liar would say "yes" because he knows his brother would tell the truth about him being a liar. In the second question, "If I were to ask you whether you always tell the truth, what would you say?", the truth-teller answers "yes," while the liar, knowing he lies, would also answer "no." This creates a consistent logic where the responses directly reveal the identity of the brother based on his nature of lying or telling the truth. Thus, both questions cleverly exploit the relationship between truth and deception to identify who is who!