My kid was one of the ones doing the biting and sudden burst of aggression stuff last year, so I felt that telling her that it was okay sometimes to hit back when someone else had done it to her wouldn't help. That's pretty much the main reason, though. She was able to see that sometimes the other child had had too much (provocation, silly winding up, whatever) because she knew that she got there sometimes, too, and thus "Tell $person that you need a break" fit as a logical puzzle-piece.

I dunno. If it helps at all, this year (at four) she had the capacity to tell two 3yos, "Stop fighting! Get a teacher! Someone needs to say, 'Stop!'," which made me laugh quietly later--not while she was telling me. And then I felt compelled to remind her gently that she's not a teacher and needs to let them work stuff out unless they're really hurting each other.... Round and round and round. She was really appalled by the fight, since it's two kids who rarely do so--and she's totally forgotten her own occasional biting and kicking from last year.