I just read the Wikipedia article about zero-knowledge proof/protocol at http://en.wikipedia.org/wiki/Zero-knowledge_proof. I understood the story of Peggy and Victor but I have a question. Could not Peggy just enter the cave from Path A and exit the cave at Path B, the whole process being watched by Victor, who is standing at the bifurcation of Path A and B? I did not see any problem of this plan of violating the rule of completeness, soundness, and zero-knowledge. In addition, it is definitive: Peggy can just prove he knows the magic word with one try. So what is wrong with that plan?