What exactly is the function of a rubber duck?

“Grief, it seemed, drove Voldemort out...though Dumbledore, of course, would hav…”

“Go on, have a pasty," said Harry, who had never had anything to share before or…”

“No estoy preocupado, Harry - repuso el anciano con tono más firme, pese a que e…”

“Professor McGonagall had a way of making him feel he must have done something w…”