Who is the Tooth Fairy Anyway?

Mariah has her first loose tooth. She is very excited for it to fall out - which led to this conversation tonight (and a Bernstein Bears movie she saw one time). We have never mentioned the tooth fairy to her - our poor confused first child.
Matt- What happens when your tooth falls out?
Mariah - I think you put it under your pillow and someone comes - a fairy or something - actually it is a tooth fairy because she has teeth to eat and talk and stuff. She has a wand. She turns your teeth into something else.
Matt- What does she turn your teeth into?
Mariah - Let's wait and see! Oh, I remember, it's small and round an shiny. I think it's a dime - no I think it's a DIAMOND! Maybe it's just a blessing from Heavenly Father. Let's wait and see.