3 yr old asking 'whats a girl?' what do you say?

You guys do make me laugh

I always answered questions simply and left it at that. If they want more you can go further. My mum (a nurse) went into Fallopian tubes and the whole nine yards - I was a bit

You'd go into DNA with a three year old Midnight? Really?

COme on OP!

Seriously?!

Tell him gender is an evil societal construct

At that age I'd go with anatomy and just make it clear that girls and boys can do exactly the same things and like the same toys. If really pushed I think I've said things like "yes, girls do sometimes wear dresses but if a boy wanted to wear a dress that's okay too" or "yes, boys often have short hair but not always".
I didn't introduce the concept of trans people till mine were older. Explaining to DD that a transmasculine friend of ours "used to be a girl" (& yes I know that's not the right language but it had to be child-friendly) - her face was an absolute picture!

I will explain (simply) about DNA and that girls are two x and boys x y. Then show the differences that brings - anatomy, growing up to carry children (women) or not (men), differences developmentally etc. Emphasise that both boys and girls are important to our world and look at male/female other animal species. I have one of each though and they are already aware that there are differences between them (not through anything anyone said or did, just observation). I love watching them grow and develop differently with access to exactly the same toys and learning experiences!!

I woildn't say "grow up to be mums and dads" - say "grow up to be men and women."

TBH I went with anatomical, and minimised that.

First I asked why DS2 was asking, and what he thought - he wasn't really sure, it seemed to be that he'd heard these words. (DS1 at a similar age had said long hair and eyelashes). Then I told him that the only difference between boys and girls was that boys had willies and girls had a vulva (nod to MN there).

Later on in the car his older brother (DS1, 6) took the conversation a bit further, and pointed out that only girls can have babies, and we discussed how women have a little pouch inside their tummy where they grow a baby, and a special hole that the baby comes out of (which stays squeezed shut most of the time, so no, you can't look at it because there's nothing to see).

I'm very strong on the idea that there are no girl/boy colours/toys/clothes and everyone can do whatever they want, and any kid that tells them otherwise is being silly.

'Girls are children who grow up to be women, like mummy. Boys are children who grow up to become men like Daddy' (or other suitable male if there is no DF in his day-to-day life). I think your line about boys and girls are all children is good, and can later be reinforced by saying that children can play how they like and do what they want regardless of whether that are boys or girls.

There is no need to go into every possible permutation of sex or gender at this point. It's most likely to be normal curiosity which comes with that tiny bit more independence of going to nursery, as he may not have interacted that much with groups of children by himself before.

A young female human under the age of 18.

Sounds correct to me. You can say girls become women and boys become men. Girls can grow up and become mothers and boys fathers. You could say some (but not all) people dress boys and girls differently but there is no actual, practical reason why they should.

It's a very vague question, probably needs some investigation as to what he actually means. My toddlers came out with incredibly profound sounding questions sometimes, but they always just meant something pretty straightforward when I figured out what they meant.

But I'd probably go with what you've said, boys and girls are children, the same except their bodies are a bit different. Anatomical differences are the only thing you've got really.

Could you point out the other girls in his life? Eg mummy, nanny, auntie etc?