"Parents," says Junia. "You have a set of parents, and your parents each also have two parents, and so do theirs, and so on. And each of those pairs of people met somehow or other and decided to have children, with or without the intervention of the divine, but mostly without, on the whole. And for there to be another you, there has to be another of your mother, and another of your father, and another of each of their mothers and fathers - doesn't there? And how can all those people come together just the same even living in entirely different places and - and Isabella's an angel, but none of them were - why does this work so that people who are together in one universe tend to be together in all the rest?"