When a fight you're losing suddenly gets turned around

Me: Stand up straight, you're getting an old man paunch. Adam: Well, if I am it's all your fault. Me (incredulous): How is that? Adam: Because I work hard all week and come home and don't even get a massage. So, my back stays sore and I slouch and it's your fault. Me: OK, so if you get colon cancer it'll be my fault because I didn't stop you eating a serve of red meat every day. Adam: Yep. Me: But what about all the good stuff I do, like serve up your vegetables. Shouldn't I then deserve the credit for extending your lifespan by whatever increment I am? Adam: No, because I made the choice to eat said vegetables. Me (desperate for a point): Then I won't cook them anymore then. Adam (putting his hand behind his head, in the manner of the self-satisfied): (smugly) Fine by me then. Me: I thought we were talking about your back. Adam: No, you were. Me: Just forget it. Adam: Can I have that massage now? Me: No. Adam: Please? Ha! Check-and-mate!