It's the face the says "Maaahhm".
As if to say "Mom (really drawn out) are you kidding me?"
It's the "I am so annoyed that you are even talking to me" face. I am really not sure of any other way to explain it, but I know that some of you must have seen it on your own little ones faces, huh?
Here it is....
See, her lips are in the ready position to make the Mmmmmm sound!