Oh boy, Romney's on a roll now with the birther jokes. At a rally this morning, Romney: "Knock knock." Crowd: "Who's there?" Romney: "Obama's birth certificate." Crowd: "Obama's birth certificate who?" Romney: "Exactly!" Then, "Obama's birth certificate walks into a bar and says, 'give me a drink, I have a frog in my throat.'" Bartender: "Did you say 'a fraud in your throat?'" And: "Take Obama's birth certificate. Please!" Finally, advisers pulled Mitt off the stage in the middle of "Obama's birth certificate just flew in from Kenya and boy, are his --"