I was with my sister when my 3 year old niece came into the room minus her knickers, holding up her dress and pointing downwards, demanding 'What's this called?' My sister was speechless, she obviously hadn't expected that situation to arise quite so soon! She said, playing for time,
'Well, it hasn't really got a name that you could use or remember, but when you are a big girl it will be called a Vagina'. 
BIG mistake - every half hour after that for about the next five years my niece referred to her 'My Big Girl's Gina'. 