|Gather 'round the campfire|
The Tooth Fairy is
Francis Dolarhyde a mythological (or perhaps metaphorical) fairy-like creature that takes the fallen-off milk teeth from children and gives them money in exchange.
 The story
The myth usually consists of the following.
- Child loses their baby tooth.
- Child is told to put their tooth under their pillow.
- During the night the tooth is replaced by a coin or maybe even a bank note if the child is very lucky.
This is said to be the work of the Tooth Fairy. This is a fairly solid explanation, as you sleep on your pillow, how could someone reach in and take the tooth without waking you up! It must, therefore, be magic.
 The Tooth Fairy as a symbol
The Tooth Fairy story is basically a simple piece of mythology told specifically to children to entertain young minds and convert a painful or frightening experience into something fun and exciting. It's generally seen as harmless fun, on par with Santa Claus and not some heinous and dangerous practice.
Because believing in the tooth fairy requires child-like logic and thinking, the Tooth Fairy has been developed into a kind of insult: e.g., "you'll be saying the Tooth Fairy did it next" to display incredulity. It is also often invoked by atheists in some reductio ad absurdum arguments; e.g., "if atheism is a religion, not believing in the Tooth Fairy is also a religion".
 Tooth fairy science
The nature of the tooth fairy is generally known and understood by Western adults; for this reason it has been adopted as a useful metaphor for studies into the fundamentally unverifiable, under the label "tooth fairy science", coined by Harriet Hall of the Science-Based Medicine project who described it thus:
“”You could measure how much money the Tooth Fairy leaves under the pillow, whether she leaves more cash for the first or last tooth, whether the payoff is greater if you leave the tooth in a plastic baggie versus wrapped in Kleenex. You can get all kinds of good data that is reproducible and statistically significant. Yes, you have learned something. But you haven’t learned what you think you’ve learned because you haven’t bothered to establish whether the Tooth Fairy really exists.
|—Harriet Hall MD|