"Yes Sir!"

This is Charlie's phrase of the moment. I have no idea where he got it from, seeing as I can not remember Mike or myself saying "Yes sir!" to anything, ever. Except today, in imitation of Charlie. Typical usage is as follows:
CHARLIE: "Daddy, will you read elephant book?"
MIKE: "Which elephant book do you mean? You want Little Elephant's Trunk?
CHARLIE: "Yes sir!"
I suppose this will serve him well if he decides on a military career.