"If you two don't knock it off, I'm going to make both of you bathe a donkey!!!"
It works. For now. Learn and adapt, man. Another tenet.
You know, I would just write up my tenets of parenting, except that every time I say "tenet", I think of Doctor Who. It's very distracting. Because then I start thinking about Torchwood, and Captain Jack Harkness...Ahem. Anyway.