Muggles' Guide to Harry Potter/Magic/Wrackspurt

Overview
Wrackspurts are invisible creatures that fly in through your ears and cause momentary confusion. As Luna Lovegood seems to be the only character who believes in their existence, we can guess that they spring from the fertile imagination of Xeno Lovegood.

Extended Description
On the Hogwarts Express in Harry Potter and the Half-Blood Prince, Luna Lovegood, seeing Harry's momentarily befuddled expression, asks, "Wrackspurt got you?" She then proceeds to explain that they are invisible and "fly in your ears and make your brain go fuzzy." She then waves her arms about her head as if warding off flies, upon which Harry and Ron look at each other and hastily begin to talk about Quidditch.

When the Trio visit Xenophilius Lovegood's house in Harry Potter and the Deathly Hallows, Xenophilius explains that one of the components of his recreation of the diadem of Ravenclaw is the Wrackspurt siphon (which looks like a golden ear trumpet), "to remove all sources of distraction from the thinker's immediate area."

Analysis
One wonders whether Xeno truly believes in Wrackspurts, or has consciously created them to explain what is known in the Muggle world as a "senior moment."