teeth
Explanation
The Joke
A child is sitting under what appears to be a bed or a bunk, reading a note aloud: "I'll be back for the rest tomorrow." The caption below reads: "I'm no longer allowed to write the Tooth Fairy notes."
The joke is that someone (presumably a parent playing the role of the Tooth Fairy) wrote a note that was meant to be whimsical but came across as deeply threatening. Instead of the expected cheerful Tooth Fairy message like "Thanks for the tooth! Here's a quarter!", the note reads as though a menacing entity is coming back to collect the child's remaining teeth -- implying forcible extraction rather than the natural process of baby teeth falling out.
The Humor
The humor comes from the gap between the innocent, beloved children's tradition of the Tooth Fairy and the horror-movie interpretation that the note inadvertently creates. "I'll be back for the rest tomorrow" is the kind of thing a villain in a thriller would say, not a magical fairy leaving coins under pillows. The comic plays on the idea that some parents have a sense of humor that is wildly inappropriate for the audience, and the caption -- delivered as a deadpan admission of consequences -- suggests this person has been banned from Tooth Fairy duty after traumatizing the child. It is a compact joke about how easily a well-intentioned gesture can become terrifying with poor word choice.