The secretary at a computers company is pregnant! It may have been one of the three partners' fault, or maybe someone else's. If a partner was the guilty party, he knows it... but he doesn't want the others to know it was him.

The three partners agree that they must know whether it was one of them who made the girl pregnant (thus possibly allowing a suit against the company) or if it was someone else.

How can they decide if one of them is to blame, without anybody having to accept being the father, if that was the case?

(Some notes: They might just write on papers either "I DID IT" or "I WASN'T IT", but the handwriting might give the guilty party away. Putting a white or black marble in an urn (white=innocent, black=guilty) might work, but someone could possibly see what color was being put in; also, if the guilty one went first, the second could peek inside and realize the answer. The optimum solution should not require much --or any-- extra equipment, and should "resist" inquisitive partners. And, of course, being all of them quite capable hackers, computers are out of the question!)

Each one writes a YES or NO on a piece of paper, as he pleases; YES doesn't mean "I did it".<p>Then, they form in a circle, and each one shows his paper to the one to his right.<p>Finally, each declares if the paper he saw had the same word as he had written. However, if one of the three was the father, he is to INVERT his answer, and say "the same" if the words differ, of "different" if the words are the same.<p>If nobody is the father, the answers should be either "the same" three times, or "the same" once and "different" twice.<p>However, if someone was the father, there should be either "the same" twice and "different" once, or "different" three times.

Posted by e.g. on 2004-02-23 14:49:09