Muggles' Guide to Harry Potter/Magic/Flobberworm

Overview
The flobberworm is a large worm that can grow up to ten inches; each end is indistinguishable from the other. It lives in damp ditches, and its preferred food is lettuce.

Extended Description
In Harry Potter and the Goblet of Fire, Vincent Crabbe claims to have been bitten by a Flobberworm; this information is provided to Rita Skeeter who is, at the time, gathering damaging information about Rubeus Hagrid. However, this is rather damaging to the eventual story, because in fact Flobberworms have no teeth.

In Harry Potter and the Prisoner of Azkaban, after spending all term teaching about Flobberworms, and that they will die if fed too much lettuce, Hagrid manages to kill all the Flobberworms he has been teaching with over Christmas break by feeding them too much lettuce.

Analysis
The key aspect of the flobberworm is its total harmlessness. As Care of Magical Creatures teacher, Hagrid is very uncertain of himself, and his first-ever lesson goes rather horribly wrong, with a Hippogriff injuring Draco Malfoy. In order to avoid a repetition of this problem, Hagrid retreats to the safety of the innocuous, and so his class are faced with a long series of boring lessons about the flobberworm. The main purpose of this seems to be showing the fragility of Hagrid's self-image; big and fearsome as he is, he is easily upset and easily dismayed.