Dad (dy) and Mom (my)

Somehow over the past week or so Isaac has started calling David 'Dad' instead of 'Daddy'. I for the most part am still mommy with the occasional mom. We are trying to figure out where it came from, or where we picked it up... but it does make us chuckle when he says it. "Dad... where are you. Dad, come here. DAD!" It sounds too grown up for our liking, and we know it will swing in and out of dad (dy) for a long while, but for now just another reminder that he is growing up all to fast.