a prisoner escapes prison and reaches a place where there are two doors before him,each guarded by one man.the prisoner knows that one of the doors lead him back to prison and the other leads to freedom.he also knows that one of the guards is a liar and the other one is truthful,but he doesnt know which one is the liar.which ONE question can he ask to either of them which will surely show him the path to freedom.I WAS STUMPED BY THIS ONE,PLEASE HELP ME OUT...