"Extra's" AJ Calloway met up with America's greatest Olympic athlete, Michael Phelps, to talk about competing in the upcoming Summer Games in London.
The gold medalist is teaming up with Visa for their "Go World Campaign."
He’s encouraging fans to send their support. "A hundredth of a second can make a big difference, so being able to have a couple of extra people cheering you on can definitely help."
The London Olympics begin July 27 on NBC.