Crossing the line

Spawn X approaches me in an overly friendly, affectionate manner, given the conversation we last had and which he has not yet apologised for. The conversation, 24 hours previously, started out friendly enough and involved normal banter, in-family jokes and mild insults. But the conversation was long and there was an undercurrent of Nogadgetwednesday frustration which eventually crumbled into perpetual answering back. Think kid across the street from the McCallisters in HomeAlone, but instead of incessant questioning about the mileage, speed and 4 wheel drive capability of the airport shuttle bus, we had sarcasm. Yay. It might have been fascinating to watch from a sociological point of view, but by the time it got truly interesting I was barely hanging onto control of my own mouth to analyse the situation in a detached, reflective way. However, I am an adult. I possess the filters which Spawn X lacks. I therefore did not verbalise the many comments swirling in my head but dried the dishes with Spawn Y (both of us chanting la la la la la la until I became dizzy), while co-parent Z fielded the comments and comebacks. It was not a fun tea time.

X: I love you, mum.
Me: I love you too, mate.
X: Can I go on the computer?
Me: (Look of incredulity) Are you... forgetting something perhaps?
X: I'm sorry.
Me: (More of the Look)
X: I really am sorry.
Me: For what, exactly? I mean it's really nice to hear from you and everything, but I just want to be clear on this.
X: For being, as you so eloquently phrased it 'A total git'... And oh look you're smiling because it was quite funny- isn't it amazing that I can make you laugh about this?
Me: Yes, your gift is indeed your curse.
X: No, it's not my curse, its-
Me: No, no, no! This really is your curse. You're smart. You're witty. You're fun to be around. And you have NO idea where the line is.
X: See - I think I do know where the line is- I just don't care about crossing over it.
Me: But then this happens - so you need to learn to care. How can we stop things going there? We need a new gesture...

(We used to have a discreet 'You're crossing the line NOW' gesture that I could use on him in company without saying anything, but it backfired on a couple of occasions and lost effectiveness. We spend a ridiculous 60 seconds trying out new gestures, none of which could be used discreetly in the company of other people and one or two that could be construed as mildly offensive).

Me: None of these are working.
X: Hmmm. I shall need to give this some thought. I'll sleep on it and get back to you.
Me: That would be fantastic.