Once you’ve completed your Certified Hypnotist training course, put your girl into trance.
Tell her to forget the number 7. Suggest that it doesn’t exist.
Once you’ve brought her out of Highly Suggestible State, have her count her fingers. She’ll count them over and over again knowing that it’s impossible that she has 11 fingers.
Tell her, “I knew you were dumb, but I didn’t think you were that dumb.”