Having a newborn and a 5 year old in the house brought this one back...
When Little Man was born, Little Miss was 6.
I was changing his diaper one day and she was looking inquisitively at his parts just over my shoulder. Recognizing her curiosity we started talking about private areas of our bodies and wanting to always use proper language I told her that Little Man had a penis but that it was his and not to be bothered by anyone.
A few minutes went by when it dawned on me she might not know what her parts were called. So I asked "Do you know what your private area is called?"
Without skipping a beat and almost dismissively she says...
"Oh mine? Mine's just a regular."
And let me tell you, that just made the word vagina sound so ridiculous it took me a few days to circle back and correct her.