My Sainted Favorite Mother had a saying for as long as I can remember. If you were stating the painfully obvious or just trying to tell her something she already knew, she would lovingly snap "Teach your grandmother!" in lieu of the less polite and less cultured "Shut the hell up."
Or so I always believed. "Teach your grandmother", in this context, had a very obvious etymology: one's grandmother, it is assumed, is/was a wise, intelligent and experienced person, and any life lessons you might wish to impart to her have probably already been learned.