How to make a class keep quiet

One guaranteed way to kill a debate is by participating in it. At least, that's what's happened to the "education" thread in my kids' blogs. Try to be too clever, and they don't know how to respond after that. So I'm holding my peace in the thread on Law and Society, quietly observing as more and more of them start contributing to the discussion. It's fun to watch them pitch their theories at each other as they work out among themselves whether the Law makes Society, or if it's Society that makes the Law. When they get sufficiently confused and ask for my input, then I'll throw in my two cents. I have no wish to end yet another promising webchat prematurely.