Exchange between myself and my 3 year old when she started calling me by my first name:
ME: how come you're calling me Sara now? You know you are the only person in the entire world who can call me 'mommy'?
HER: I know. But I can teach you how to be Sara.

Well, I had no good response to that, because really, she is teaching me lol. For the record, she still calls me mommy now, the name thing appeared to be a phase, but I didn't insist either way.