teacher: (immediately nods and writes "red hair" on board) yup, that is one of alex's properties, sure.
another kid: brown eyes!
teacher: (nods and writes on board)
yet another kid: freckles
teacher: all great answers, (writes on board) anything else?
Lets call him Bob: Shes a girl
Teacher: (Pauses, looks at me, then back at the kid) Well it's not my place to confirm that, besides, you have no idea whether Alex is a girl or not.
Bob: Well she's got boobs
Teacher: (Doesnt even get mad at the smartassish response, just nods calmly) Yes, now, having boobs is a property. Gender on the other hand is much less physical, and a lot more mental. Just because she has body parts that are recognized generally on females, does not mean that she's female herself. So why don't we say, instead of "she's a girl" we'll just say she has boobs