" Perhaps,' Taran said quietly, watching the moon-white riverbank slip past them, 'perhaps you have the truth of it. At first I felt as you did. Then I remember thinking of Eilonwy, only of her; and the bauble showed its light. Prince Rhun was ready to lay down his life; his thoughts were for our safety, not at all for his own. And because he offered the greatest sacrifice, the bauble glowed brightest for him. Can that be its secret? To think more for others than ourselves?'

That would seem to be one of its secrets, at least,' replied Fflewddur. 'Once you've discovered that, you've discovered a great secret indeed--with or without the bauble. "

― Lloyd Alexander , The Castle of Llyr (The Chronicles of Prydain, #3)