Of course it was Clinton.
While visiting England, George W. Bush is invited to tea with the Queen. He asks her what her leadership philosophy is. She says that it is to surround herself with intelligent people. He asks how she knows if they're intelligent.
"I do so by asking them the right questions," says the Queen. "Allow me to demonstrate."
She phones Tony Blair and says, "Mr. Prime Minister. Please answer this question: Your mother has a child, and your father has a child, and this child is not your brother or sister. Who is it?"
Tony Blair responds, "It's me, ma'am."
"Correct. Thank you and good-bye, sir," says the Queen. She hangs up and says, "Did you get that, Mr. Bush?"
"Yes ma'am. Thanks a lot. I'll definitely be using that!"
Upon returning to Washington, he decides he'd better put the Senate Majority Leader to the test. He summons Bill Frist to the White House and says, "Doc, I wonder if you can answer a question for me."
"Why, of course, sir. What's on your mind?"
"Uh, your mother has a child, and your father has a child, and this child is not your brother or your sister. Who is it?"
Frist hems and haws and finally asks, "Can I think about it and get back to you?" Bush agrees, and Frist leaves. He immediately calls a meeting of other senior Republican senators, and they puzzle over the question for several hours, but nobody can come up with an answer. Finally, in desperation, Frist calls Bill Clinton and explains his problem.
"Now look here Mr. President, your mother has a child, and your father has a child, and this child is not your brother, or your sister. Who is it?" Clinton answers immediately, "It's me, of course, you dumb ***."
Much relieved, Frist rushes back to the White House and exclaims, "I know the answer, sir! I know who it is! It's Bill Clinton!" And Bush replies in disgust, "Wrong, you dumb ***, It's Tony Blair!"