When Simone Biles prepares for a routine, chalking up her hands, there's an incredible amount of pressure on her 4-foot-9 inch frame.
A five-time Olympic Medalist, Biles is one of the best gymnasts of all time. At her first Olympic Games in Rio, she took home four gold medals and one bronze.
She even has a move named after her: "The Biles." Commentators have called it "unreal."
But for Biles, stepping onto the mat means shutting all of that out.
"Most of the time before I take the mat, I just tell myself, 'Confidence' and 'You have this,'" Biles tells CNBC. "It's like a daily reminder."
Her new book Courage to Soar reveals how the Olympic champion had to learn to believe in herself from a young age. She grew up in foster care. Later, she had to make the decision to forgo high school and the rest of "normal life" for training, and frequently she had to ignore critics and skeptics of her road to Rio.
But those challenges ended up helping her.
"Every challenge I'd ever faced has made made me stronger and more determined," Biles writes.
She tells CNBC that some of the best advice she ever received was from her sports psychologist.
"He always tells me […] 'Don't take anything for granted. And don't listen to what everyone else says to you, because once you go on the floor, you're the only one that can control what you do.'"