"Not Fair!"

First of all, where do children learn this word/concept/retort?

Second of all, here's the twist on fair 'round these parts:

The other day I dropped a DVD on the ground while I was holding the baby and putting another DVD in the player. Both of the girls saw me drop it. I said, "Can you get that for me, please?"

Gianna jumped up and said, "I do, Mama."

Stella responds, "Hey, I didn't get to get that!"

I say, "Well, yeah, you just sat there. You didn't want to help me."

She says, "Well, next time it's my turn to help."

"That's fine. But we don't take turns helping. We just get up off our keisters and do it."

"Well that's not fair."