Riddle: You go and see two roads:one way to heaven and one way to heck. You see two men guarding there. One always tells the truth, and the other always lies, so if you ask who always tells the truth, they'll both say,"Me!" However, they know who always tells the truth and who always lies. You can only ask one question to both of them, and you can't guess. How can you figure out which way is heaven?
Answer: You aak both men,"What would the other person say if I asked which way is heaven?" Now let's call the truthteller ManA and the liar ManB. Now ManA knows that ManB will lie, and he also knows he'll point to heck(because he lies), so he'll point to heck. ManB knows that ManA will tell the truth, and he also knows that he'll point to heaven(because he tells the truth). However ManB always lies, so he'll point the other way, which is heck. Then you know the other way is heaven.