A police inspector wants to solve a theft and is close to tracking down the thief. Reconstructing the crime, he needs to follow the escape route, and wonders which of two possible paths the thief had selected. In order to clarify this, the police inspector interviews three witnesses. Therefore, he is allowed to ask two questions, each question to one person. Among the witnesses are a notorious liar, a totally honest person, and someone who lies on occasion. The police inspector does not know which of the three witnesses is always ying, occasionally lying, or always telling the truth.
How should the police inspector proceed in order to find out which escape route the thief took? What questions should the inspector ask?
The first question to ask a witness is:
Which of these other witnesses is more likely to tell the truth?
The answers have to be divided into three possible cases:
Looking at the answers, you see that the "occasional liar" is either the questioned witness or the one to whom the other witnesses point to. Meaning, the person who is not questioned or pointed to by the other witnesses cannot be the "occasional liar". The person always says the truth or the untruth. The police officer now asks a second, more complicated question to this particular witness:
Which escape route would the witness who does not occasionally lie indicate ?
The following two cases are possible:
In both cases, the interviewees point out the incorrect escape route. Now, the police inspector simply has to choose the other path to successfully complete his investigation and catch the thief.
To solve this riddle, it is necessary to ensure that the first question eliminates the possibility that the second question is addressed to the "occasional liar". If you have identified a person who always behaves the same way (regardless of whether he always lies or always tells the truth), then you can solve the riddle with the help of an appropriate second question.