You can say what you really feel as long as it’s to the back of their head. For let’s face it they can’t handle the truth or even a joke. So you continue the conversation even as they walk away with their back to you. For it doesn’t really matter if they understand as long as your oxygen doesn’t truly hit their ears. It might not be the best solution but it’s one that keeps you sane and them happy as only the back of their head really understands.