A prisoner is in jail. There are two doors, one leads to freedom one leads to death. There is a guard at each door. One guard always tells the truth, the other always tells lies. The prisoner is allowed one question to either of the guards.
What is the question that will take him to freedom ?
Answer: If I was to ask the other guard which door leads to freedom, what would he say? Whichever door the guard points to, you go out the other door!