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The two guards.
Let T be truth teller, and L liar; Q is a question, A is an answer.
Q: G2, what would G1 tell about the truth of your statement if you tell me whether this path is safe?
If G2 is T, and path is safe, A: False.
If G2 is T, and path is not safe, A: False.
If G2 is L, and path is safe, A: True.
If G2 is L, and path is not safe, A: True.
Therefore, the answer differentiates truth teller from liar. I only need to refine the question so that it also distinguishes safe from non-safe path. Let T be truth teller, and L liar; Q is a question, A is an answer.
Q: G2, would G1 advise me to trust you if you tell me that this path is safe?
If G2 is T, and path is safe, A: No.
If G2 is T, and path is not safe, A: Yes.
If G2 is L, and path is safe, A: Yes.
If G2 is L, and path is not safe, A: No.
Huh :-( You stand at a fork in the road: one path leading to death, other to home. Next to each path there is a guard: one always tells the truth, the other always lies, and you can't know which one is a liar.
You can ask one guard one question to find out which path leads to heaven. What question will guarantee your safe journey to home?
