Suppose there are twin brothers; one which always tells the truth and one which always lies. What single yes/no question could you ask to either brother to figure out which one is which?
Answer: Would your brother say that you tell the truth?
The key to this logic problem, is to find a question that the two brothers would answer differently, and that difference would therefore identify the two from each other. The lying brother would answer the above question "yes." The truthful brother would answer the same question "no."