DS has just started screeching, sometimes as a sign of frustration (as in 'STUUUUCCKKKK'), but also just for the hell of it (as in 'MIIIINNNEEEE'). I can't decide how to deal with it. Should I just ignore it, ie not even help him unstick whatever has got stuck? Say 'no'? I've tried explaining (in a quiet, calm voice) that he doesn't need to scream, but don't think he really knows what I'm on about...