"J, honey, we don't whack dragons with rolling pins when the dragon is your sister's head."
Another oldie...from back in J's early days of constant climbing/teetering on edges of dangerously high places:
"J, we SIT on tables, not stand on them."
(Ummm...actually, polite people don't sit or stand ON tables, but I could tell that to my son until I was blue in the face with the same result each time. That result being no indication that he heard me or cared if he was polite or not.)