Millie loves singing her heart out to Queen's "Bohemian Rhapsody" in the car.
But just don't try to join in with her. She won't appreciate it, because she takes her performances very seriously.
But every time her mom, Stephanie O'Donnell, tries to harmonize with her, she screams out "no." "Why can't I sing with you?" her mother asks. "No," Millie replies. That's her told.
It's unclear exactly where the clip was filmed, but O'Donnell posted it to YouTube last month and it's now going viral.
"Trying to clear some of the videos off my phone I just found this video of Millie singing her favorite song back in June 2015," she wrote. "And of course, not letting anyone sing with her."
See how Millie's version compares to the original here: