The Queen's Riddle

Barack Obama met with the Queen of England. He asked her, "Your Majesty,
how do you run such an efficient government? Are there any tips you can give me?"
‎"Well," said the Queen, "the most important thing is to surround  yourself with intelligent people." Obama frowned, and asked, "But how do I know the people around me are really intelligent?"
The Queen took a sip of tea. "Oh, that's easy; you just ask them to answer an intelligent riddle." The Queen pushed a button on an intercom. "Send David Cameron in here, would you?"
Cameron walked in and said, "Yes, Your Majesty?" The Queen smiled and said, "Answer me this please, David, your mother and father have a child. It is not your brother and it is not your sister. Who is it?" Without pausing, Cameron answered, "That would be me."
"Yes! Very good," said the Queen.
Obama returned home and asked Joe Biden, the
same question: "Joe, answer this for me. Your mother and your father have a child. It's not your brother and it's not your sister. Who is it?" "I'm not sure," said Biden. "Let me get back to you on that one..."
He went to his advisers and but none could give him an
answer.  Finally, he ran into Paul Ryan on Capitol Hill. Biden asked: “Paul, can you answer this for me? Your mother and father have a child and it's not your brother or your sister. Who is it?"
Paul Ryan answered: "That's easy, it's me!"
Back in the Oval Office, Biden told Obama: "I did some research and I have the answer to that riddle. It's Paul Ryan!"  Obama leaped to his feet and yelled: "No, you idiot, it's David Cameron!"
[Thanks ECS]