Wanna know the REAL difference between boys and girls?

I have learned from my 4 year old the REAL difference between boys and girls. We've had her friend Max over this week, so I guess they've been discussing it- drum roll please...... Girls have lips on their mouths, and boys do not. Yup, she informed me of this on our way to playgroup this morning. I tried to convince her otherwise- I used logic and reasoning, but nope, she refused to believe me. I even told her that mommy knows more than her and so she should be believe me. She informed me that I was wrong about that too. Oh well.