8. "Mommy, what are tooth faeries like, and what do they do with all the teeth?"
Whatever you may have heard about tooth faeries, it's probably a lie. That's not to say there isn't a fragment of truth in some of the stories...but none of them get the whole thing right.
Do they collect teeth? Well obviously this bit is generally correct, hence the name.
Are they faeries? Not all of them. Some are, and it may be these individuals who started the legends, but there are very few faeries who still attend these duties...more about them shortly. Most tooth duty is done as penance for committing a crime against the courts of the fey. Punishments from the Seelie court require service to a fey noble during the day, while a specified number of baby teeth (often in the hundreds) need to be collected from the young at night. Punishments from the Unseelie court require infecting healthy adult teeth, where the pain from rot and decay fuels specific dark fey charms (such punishments often require the infection of thousands of teeth, beginning with those in the mouths of loved ones).
What do they do with all the teeth? Teeth taken as penance for the Seelie Court are elaborately crafted into sets of false teeth for the lineages of fey nobles who have become toothless due to excessive inbreeding over countless generations. The voracious appetites of fey nobles means they often go through a set of false teeth annually (if not faster), hence the need to outsource their tooth gathering. The creatures once known colloquially as "Tooth Faeries" now operate as collection agents and crafters, so that the nobles don't have to deal directly with criminal riff-raff.
Teeth infected for Unseelie penance are left to rot in the heads of their victims for as long as possible. Only once they are removed by dentistry (professional or amateur) does the conduit of pain energy become severed. It is said that residual pain may be stored in them, and they may be used as batteries for Unseelie amulets, but it is generally considered more effort than it's worth to harness the low energy within these rotten fragments.