Riddle: There's a King and he has to save his kingdom.
There are 2 paths. 1 takes him to his kingdom the other takes him to a Dragon who will eat him. There are 2 brothers: 1 that always lies and 1 that always tells the truth. You don't know which one is the liar and which is the truth teller. What question do you ask them both to figure out what road you take to get to your Kingdom.
Answer: You ask them "What road would you tell me to take if you were your brother?" The liar knows his brother would say the right road (to the kingdom) and would lie and tell you the wrong road to take. The truth teller knows his brother always lies and that he would say the wrong road so he tells you what road he would say. (which is the wrong road)
Therefore you would take the road they didn't say.