"Since they were told to raise the hand when they see red, X and Y had red and Z had black, X realised he had red cap because Y was raising hand when Z had black cap, he was the one left with red cap which Y saw." Correct, Ritthe_jasus! Ok, now what if all of them had red caps, how'd x find he is wearing red cap? Same scenarion, only this time all of them with red caps. Dare it? :P