Simone Biles can't be replaced: If the gymnast drops out of her individual events, here's what will happen

Simone Biles pulled out of the gymnastics team finals in Tokyo this week, a decision she said left her feeling "super frustrated." Biles said she was physically fine, but decided to withdraw due to mental health reasons. There are still several gymnastics events left, and Biles could return. But if she doesn't, there are some events that no other U.S. gymnast can compete in. Here's why. 

Biles pulled out before several Olympic events occurred. The first one she missed was the team finals, in which gymnasts from each team complete in each of four events: vault, balance beam, uneven bars and floor exercise.