I'm clueless with this stuff...how do you answer this question from 3-year old DS? I'm pregnant with our 2nd which has triggered the questions but really didn't expect to have to be faced with these when he was so young! He's bright & curious so I'm used to this to some extent - he already asks questions about why he has a 'wee man' and I don't, and boobs etc but this is more of a head scratcher 
He asked why are we getting a girl baby and I said that sometimes you get a boy baby and sometimes a girl baby - you don't get to choose, just depends what is made in mummy's tummy. Then it was 'How are they made?'
Help!