tutoring and teeth

within five minutes of sitting down with my student, a first-grade girl, to read a book, the following conversation ensues:

"alright kimberly, what book do you want to read today?"
"let me see your teeth?"
"what?"
"show me your teeth. go... ike deeeez."
"okay. cheeeeeese."
"you have dirty teeth. you don't brush your teeth."
"what? yes i do!"
"let me see again."
"cheeeese."
"no you don't. you have brown spots. you don't brush you teeth."
"yes i do, kimberly! i brush twice a day!"
"then why are you teeth dirty if you brush your teeth?"
"look little girl. i don't know why, but i have excellent oral hygiene, okay! i brush twice a day, and i floss. maybe once you grow up and get braces when you're in college and then start drinking coffee and tea, you'll realize how hard it is to keep you teeth completely white! now... let's get back to picking a book."
"okay. . . but you're teeth are still dirty."