Fallon kicked things off by asking the star swimmer about one of the most iconic images to come out of the Rio Games -- Phelps' famous "Angry Face."
"I was sitting in the ready room and I remember [thinking], as I'm making a face, 'Yup, that's on camera. Someone will pick that one up tomorrow,'" Phelps said of the moment that became a viral hit.
Fallon also brought out a surprise for the superstar: A life-size cardboard cutout of Phelps looking hilariously enraged. He joked that the cutout was freaking him out, adding that if he was a competing swimmer, the look would have made him think, "Not only is he going to beat me at swimming, he's going to murder me."
The 31-year-old new dad, who was also promoting his water safety and healthy living organization, The Michal Phelps Foundation, opened up about his experience in Rio, where he was joined by fiancee Nicole Johnson and their 3-month-old son, Boomer. According to the Olympian, his time outside the pool was peacefully uneventful.
"As soon as I got done swimming, I checked into the hotel with my family and left like a day or two days later," Phelps explained.
After quickly changing the subject, Fallon brought up the only topic of conversation Phelps loves more than swimming, his infant son. The host asked the question many fans have been wondering for some time: Why the name Boomer?
"We didn't want a normal name," Phelps said, laughing. After convincing his fiancee about the moniker, there was no turning back. "We really had no second option whatsoever."
"He's definitely going to be the cool kid," Phelps added, understating things a bit.
Later, the swimmer and the host went head to head in a game of Egg Russian Roulette. While he has no fear facing off against the world's toughest competitors, Phelps seemed more than a little hesitant to smash what could be raw eggs into his hair.
After admitting he was "nervous," Phelps fought through the trepidation and ended up with a bit of egg on his face, quickly losing to Fallon by drawing two raw eggs in a row. Fallon can now say he bested the world's greatest Olympian (sort of).
"You’re going to bring tears to my eyes now," Phelps told NBC’s Michelle Tafoya when asked about what it felt like to have his son watch him win gold. "I wanted to be with him more. Being able to share that one moment that I had with him, [that was] definitely something that was very special." Check out the video below to hear more.