who trained who?

Once again, out of the corner of my eye, I see Nahum rolling over on his back and looking over at me. It's only the fourth time in the last hour. Upsidedown Nahum = getting tummy rubs, now an axiom. How this came to be is what got me wondering.

Who trained who? I want to say that I taught Nahum that whenever he rolls upsidedown and looks at me that he gets his tummy rubbed. Although, I suppose it could have worked the other way too. Perhaps I learned that whenever he rolls over he needs to have his soft tummy hairs petted. I am not really sure who started this or who's in-charge of this exercise now. It seems to happen more and more frequently as time goes on. I am not complaining though and I haven't heard him complaining either - so maybe the question is irrelevant.