So much sass for a 3 year old

Where exactly does a 3 year old girl's attitude come from? Her sitter had quite a time with Erin the other day. Another child, Aubrey, is about the same age as Erin and they have similar interests--you know, dressing up, playing with dolls, undressing, playing with stuffed animal, re-dressing, playing school, and dressing up. Oh, and dressing up. Anyhow, these two little ladies love each other but fight like cats and dogs. Apparently on this particular day they really got on each other's last nerve. One would proclaim that she was no longer her friend. This would prompt a retaliatory gesture such as hitting or taking a toy away. Then the offended child would threaten to "call mommy" and tell of the other girl's bad deeds--which apparently they would then do via fake cell phone. As the caller would line out all of the other girl's bad deeds, the other girl would sob and talk about how hurt her feelings were. Surely the sitter would have gladly taken a bullet to avoid hearing this constant go-round. She finally did get sick of it and told the girls that the next one to act up would get a REAL call to Mom or Dad. Aubrey immediately turned pale. Erin's gleeful response: "You can't call my mommy and daddy. They're at work."