You've heard of trading selfies? 

Well, UFC superstar Demetrious Johnson trades smellfies with his wife ... but there's actually a good story behind it. 

We were talkin' to Mighty Mouse about the best picture in his phone that he HASN'T posted on social media ... and his response -- "My son's poop."

Soooo ... why??

Watch the clip, Johnson has a pretty adorable explanation ... until our photog makes it weird. 

BONUS -- we also ask DJ how he feels about Henry Cejudo changing his nickname before they face off at UFC 197 this weekend ... pretty funny stuff.