"Surnames!!" joke

A woman had 8 children, all of them boys. So, one day a magazine sent a journalist to her house for an interview.
He asked her about the boys and what their names were; she said, "Johnnie".
"Right", he said, "what about that blond one over there?"
"Johnnie", she said.
"Oh, and the tall one with the freckles?"
"Johnnie", she said.
"Well, and the little chubby one with the baseball cap?"
"Johnnie", she said.
"Are all your boys called Johnnie?" he asked, "Isn't that terribly complicated?"
"Not at all", she said, "it makes everything very easy, actually. When I shout: Johnnie, tea is ready!, they all come. When I say: Johnnie, it's time for bed!, they all go to bed."
"I see. But what if you want only one of them?"
"No problem." she answers. Then I call them by their surnames."