"From their struggles to establish dominance over each other, siblings become tougher and more resilient. From their endless rough-housing with each other, they develop speed and agility. From their verbal sparring they learn the difference between being clever and being hurtful. From the normal irritations of living together, they learn how to assert themselves, defend themselves, compromise. And sometimes, from their envy of each other's special abilities they become inspired to work harder, persist and achieve."