Let's just face it, teens and adults speak a different language, and live in another country. In the adult country, sending a picture of someone's a** would be seen as a cry for help. Psychiatric hospital, open your doors. But in the land of the teenagers, any and all body parts, gross and sexual language, and sarcastic repartee is expected, welcomed, and reinforced!
A butt doesn't seem so bad to me, but a breast, that's another thing. Maybe your next talk with your teen is about acceptable body parts for photo sharing. Remember that your teen lives in the "awesome" part of the brain, which needs constant awesome experiences to keep it nourished and active. This is where the "rude and stupid" comes in. Your job is just to make sure that rude and stupid doesn't turn into mean or dangerous.
