Michael Phelps had just won his 20th Olympic Games gold medal at the Olympic Aquatic Centre, when the Baltimore native took the podium to claim his prize for beating Chad le Clos in the 200m butterfly, the event he had lost to the South African four years earlier at London 2012.

Phelps stood on the podium with his gold medal and as the USA national anthem played he burst out laughing:

He had cracked up, apparently because his friends watching from the seats had just  brought a Baltimore Orioles baseball tradition to Rio de Janeiro, chanting "OOOOOOh" for the line "Oh say does that Star-Spangled Banner yet wave." And as he walked towards the waiting photographers he could be seen mouthing. "I can't believe you just did that!"

He has shouted out the Baltimore Orioles on Twitter before. 

He's even participated in the team's batting practice. 

So it makes sense that a Baltimore tradition is the reason Phelps was laughing. Here's Phelps' explanation of what exactly was so funny. 

Phelp's outburst, while causing quite the social media frenzy, had no effect on his races the following day, however. After about five hours of sleep following his 200 butterfly and 4x200 freestyle relay victories , he was back at the pool and ready to go for his fourth straight gold medal in the 200 IM.

The American cruised through the preliminaries of the 200m individual medley on Wednesday, posting the third-fastest time behind team-mate Ryan Lochte and Germany's Philip Heintz.

Phelps said: "My body hurt a little bit" but he should be good to go for the evening semifinals. As he says, "I've been able to put my body through things like this over the years. Hopefully I can keep it rolling one last time."

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