In the Titan's Curse, chapter 17, Percy challenges Atlas to a fight. He remembers Chiron saying "Immortals are constrained by ancient rules. But a hero can go anywhere, challenge anyone, as long as he has the nerve." Then Percy says "Once I attack, however, Atlas was free to attack back directly, with all his might." He implies a demigod must attack a Titan or God before they can attack, but I am fairly sure there have been several points where the immortals attack first. Any thoughts?