THE RING OF TRUTH (Reading the mind)

If you know Sherlock Holmes, you are probably a fan of him. His ability to tell whether someone had recently come from Africa, left the Army, had sausages for breakfast, or was a recovering alcoholic has fascinated almost all of us. There is nothing supernatural about it, he simply uses his five senses to create the illusion of a sixth, or simply call it the science of deduction.

We will now use this deductive reasoning to read someone else’s mind (kind of :D ). All you need to do is, call two of your friends and give a ring (doesn’t need to be a diamond ring :P ) to any one of them. After that, you let them decide who is going to keep the ring so that, you don’t know which one is the ring holder. But, after asking a few yes/no questions (they may lie to you), you can identify which one he/she is.

Experiment:

For this experiment, the subjects should either speak the truth (honest person) or lie to you (liar). It is important that your friends know what the other person has chosen to be i.e honest or a liar. Both of them can choose to be of the same or different types. This is the single most time when the liar has to be true to the other person. You give them a ring and one of them keeps it. You have no idea of who has the ring or what each of them has chosen to be (honest or a liar).

You only need to ask two questions to them. You ask “Do you have a ring?” to one of them and “Did you both decide to be same type of person?” to the other one; these will be referred to as first and second questions respectively. Both of them should reply with a ‘yes’ or a ‘no’ to the questions. You have no idea whether the answers given by them are truthful or false since they can be either speaking the truth or lying. The only case where the answer you have received is correct is when both of them chose to be honest ,but you are uncertain of their honesty in the first place. Despite having numerous possibilities, you CAN deduce which one of them has the ring using their answers. Cool, right?

Demo:

Suppose A and B are your subjects. After you let one of them keep the ring and decide what type of person they are going to be, you ask them the two questions, one each. There are many possible cases of what they are going to answer but let us suppose one of the cases. Suppose you asked “Do you have the ring?”, to which A replied NO and “Did you both decide to be same type of person?”, to which B replied NO. You now tell them that A has the ring. Seeing you come off as correct will surprise them and repeating the experiment multiple times, and still having you guess correctly every single time will leave them unsettled.

Did you catch an idea of what happened up there? If you didn't, don’t worry. Write all the possible cases in a piece of paper and try to deduce it. If you don’t want to go through the trouble, just hop to the next section.

Deduction:

After gathering the answers from both of them, you can easily deduce who the ring is with. Let us make a table for the possible cases and deduce a conclusion.

If both A and B are honest, they’ll reply with a YES to the question “Did you both decide to be same type of person?” . If A is honest and B is a liar, A will answer with a NO while B will answer with a YES (even though they are of different kind, B will answer with a YES as B chose to lie). All the possible cases and the expected reply for the second question is given below in the table.

Observing the table carefully, it can be deduced that if a person says NO, the other person is a liar and if they reply YES, the other person is telling the truth. After you know whether the other person is honest or a liar, you can tell who has the ring.

From the above analysis, if you ask the first question, “Do you have a ring?” to A and the second question, “Did you both decide to be same type of person?” to B, you can come to the following conclusion:

Tada!