Mother: "are you going to be a good girl for Granny this weekend?"
20-month-old person: "No"
No slouch, this one.
There's no way she could promise to be a 'good girl' when given no indication of what actions that would entail.
If trying to meet someone else's expectations, we are fearful if we do not know exactly what those expectations are (and fear makes it very hard for us to do anything at all).
Even if we do understand the expectations, the general 'good girl' script allows no room for manouevre, because someone else has decided what 'good girls' do. "Sit at the table to eat your lunch like a good girl" (table manners are important, you see, even if the adults are discussing their tax returns and there's a really good cartoon on TV... /sarcasm)