We initially thought someone had told Michael Phelps they were going to beat him in his next race.
After all, what else could possibly make the American superstar burst out into such a fit of laughter?
Turns out we were wrong.
As Phelps stood on the podium after winning his 20th Olympic gold medal with a dominant performance in the 200m butterfly final on Wednesday - the likes of which we've come to expect from the biggest beast in the sport - the crowd at Rio's Aquatic Centre went nuts. It was the only reaction fitting for a once-in-a-lifetime swimmer.
He was welling up. It was difficult to tell if any saltwater escaped his eyes but the emotion was clearly there. He'd already told his junior teammates at these Games to let their emotions out - cry if you feel like crying, sing if you feel like singing.
Well, Phelps obviously felt like laughing, because that's what he did.
With the gold around his neck and the US national anthem playing, something made the 31-year-old lose it.
At first there were more questions than answers - social media wanted to know what had caught his attention. It looked like he pointed to someone in the stands and started giggling uncontrollably.
We have an explanation.
Phelps is from Baltimore. Apparently, the city in Maryland is known for being the birthplace of The Star Bangled Banner.
Fans of Baltimore's baseball franchise the Orioles have a tradition where at a certain point in the anthem - the "O" at the start of "O say does that star spangled banner yet wave" - they too yell out "O" in unison.
Whereas they might be silent during the rest of the anthem, that's the one point everyone in the stands yells out with gusto together. Head to the one-minute mark in the video below to see what we mean.
Well, some of Phelps' friends in the stands did the same thing. Sure, they might not have been surrounded by Orioles fans, but in solidarity with their mate and one of Baltimore's favourite sons, they thought they'd continue the tradition anyway.
Cue Phelps' laughter.
Competing at his fifth Olympics, there were question marks over whether Phelps would dominate like he has in the past. We needn't have worried.
He surged ahead at the start of his pet event. When it looked like he wouldn't be able to hold on to his lead, he surged again. In the final stages of the last 50 when it looked like he'd be caught by Japan's Masato Sakai (silver) and Hungary's Tamas Kenderesi (bronze), he surged for a final time.
It was enough.
Phelps sat on the lane rope with a solitary finger in the air - he was number one.
And he was number one again about half an hour later.
He anchored the US team's 4x200 freestyle relay and added another gold to his haul. Afterwards, he sat on the starting block and waved his hand in front of his throat, indicating to the world he was completely cooked.
He looked exhausted. He stayed sitting and kept his head down, only looking up to accept the well-wishes and thanks of his teammates.
They were well deserved thanks. We might never see another like him.