>>Nirman. Thanks for the endeavor. /////////////////////////////////////////////////////////////////////////////////////////////// -(1)- A said "Yes" ~Then A is Knight and and B is Knave. Because: If A is Knave, He has said "If B is a knave,then the Fountain of Youth is on this island." : Proposition X i.e. If B isn't knave, then fountain of youth is on the island. : Proposition Y /////////////////////////////////////////////////////////////////////////////////////////////// Here , you have used Proposition Y as the negation ( one having the opposite meaning ) of Proposition X. But, this is logically incorrect. The opposite of a sentence: "If A , then B" is not "If not A, then B". This might sound quite counter-intuitive but a serious comtemplation will make it clear. [ Any good book on logic will have a clarification about this. ] As for the answer to this puzzle , I won't go there right now because there can be only 2 answers: "Yes" or "No". What I want is a logically correct deduction. But that's the way to start solving a logic puzzle. So keep on trying.