What exactly is the function of a rubber duck?

“I’m very busy with schoolwork, of course.""How can she be?" said Ron in horror.…”

“Look . . . at . . . me. . . .”

“Para una muerte bien organizada, la muerte no es más que la siguiente gran aven…”

“You are the Chosen One?”“Of course I am,” said Harry calmly.“But then . . . my …”