man has to ask 2 twins a question about which direction a thief went,one twin always tells the truth,the other always tells lies,what question does he ask ?
If I asked your twin which way - what would he say?
If he is asking the one who always lies he will know to go the other way and if he asks the one who tells the truth he will also know to go the other way because he has given the liar's answer.
Does that make sense?!!