Michael Phelps explains the really awkward face he made during the Rio Olympics

People couldn't get enough of Michael Phelps' awkward facial expression during the Rio Olympics, and Jimmy Fallon decided to get to the bottom of it.

"Oh here it comes..." Phelps said on Thursday's "The Tonight Show," as Fallon repeated, "The face!"

So Phelps entertained him with a quick story about how he knew the expression would catch on even as he was making it.

"The worst is I was sitting in the ready room," Phelps recalled. "I always know there are two cameras before we walk out. I'm sitting there spitting water, like what is going on? And as I'm making a face I'm like, yup, that's on camera... Someone will pick that one up tomorrow."

Then Fallon pulled out a cardboard cutout of the "maddest face" Phelps rocked in Rio, which got some good laughs.

The "Tonight Show" host also took the opportunity to tease the Olympic swimmer about how he "didn't get the memo" regarding Ryan Loche's activities in Rio.

Watch the "Tonight Show" clip with Michael Phelps below: