Question: Tommy is in a cave. He stops to find two tunnels. There is a guard before each tunnel. Tommy knows two things: 1. One tunnel leads to the fields of punishment and the other fields of rewards 2. One guard always lies and the other always tells the truth. Tommy can only ask one yes or no question to either one of the guards. The tunnels and guards look exactly the same. What question can Tommy ask to get to the fields of rewards and not death?
Answer: "Would the other guard say that this tunnel is the field of punishment?" If Tommy asked the guard who always told the truth, the guard would say yes or no. If the guard says yes, than that tunnel is the field of rewards, because the lying guard would say yes, meaning it was no. This works vise versa yes=no. If Tommy asked the guard who always lies, and the guard says yes, than the tunnel would be the field of rewards, because the lying guard would say the truthful guard would say yes, than it is no. This also works vise versa yes=no.