@Jeannette fingers!
Deadpan face say "fingers out of your mouth" gently guide the fingers away. Then distract, redirect, distract, and distract. Repeat FOREVER! At least that's what we had to do for our girl. It worked eventually.
My nephew used to to this don’t react when he does it and try to occupied his bands either some thing else like a toy or utensil
Redirect her to a favorite toy or/and start playing with them as a distraction. Sometimes babies/toddlers do that because they or bored or it’s a comfort thing for them. She will grow out of it!
My son was doing this when no one is paying attention to him. His dad popped his hand and he stopped.
Is she using her fingers or objects?