This happened when one of my now-eight-year-old students was five or six. (I can't believe I've had some kids that long...this is the longest I've been in one place to teach.)
During his lesson, he started chatting away in an almost Joyceian stream-of-consciousness.
Me: OK, can we get back to violin now?
Him: I'm just telling you things I know!
Me: Yes, but you're telling me everything you know.