Riddle: A man dies, and he is taking the road to heaven. He comes upon a fork in the road; one leads to heaven, the other to hell. At the fork are twin brothers. Identical, except one ALWAYS tells the truth, the other ALWAYS lies. The man, who does not know which brother is which, gets to ask ONE brother ONE question. What is the question, and what path does he take?
Answer: Ask the man that lies to tell him which way hell is, and then he takes that path.That won't work because he doesn't which man is the one that lies.
Instead, he should ask "Which way would your brother say is the road to heaven?" and take the other path.
Why it works:
If he had asked the truthful brother the way to heaven, he'd point out the correct way.
If he had asked the untruthful brother the way to heaven, he'd point out the wrong way.
If he asks the truthful brother "Which way would your brother say is the road to heaven", he will tell the truth, pointing the way his brother would, which is the wrong way.
If he asks the untruthful brother "Which way would your brother say is the road to heaven", he will lie, pointing the opposite of the way his brother would, i.e., the opposite of the right way, i.e., the wrong way.
So in either case the brother you ask will point the wrong way, and you know to go the other way.