Her small world

Two little girls with their family are sitting next to me on the train. They are talking loudly and one is telling the other: "There's always a little sister and a big brother. It's never a little brother and a big sister." The other girl nods. With a triumphant tone she continues: "There's a little sister and then there's a big brother." The other one interrupts her: "No babies?" and in answer, with a firm shake of her head she says: "I said there can never be a little brother." The train stops at the station and they get off. I can still hear her as they walk away and I'm trying to remember the time that my world used to be as small as hers.