Muggles' Guide to Harry Potter/Magic/Densaugeo

Overview
Densaugeo apparently causes the teeth of whomever it hits to grow rapidly.

Extended Description
Draco Malfoy uses it first in Harry Potter and the Goblet of Fire; he had goaded Harry into dueling with him. The spell hit Harry's Furnunculus jinx and ricocheted, hitting Hermione in the mouth. As a result of this, Hermione's front teeth grew to a length of over a foot long.

Analysis
In Harry Potter and the Order of the Phoenix, we learn that combining jinxes can have unexpected effects. We can't be certain that the extreme growth of teeth was the intended effect; the spell may well have been modified by hitting the Furnunculus spell. Additionally, as this spell does not seem to be used elsewhere, it is hard to know what its normal effect would be. We can't even really tell whether the spell would cause growth of whatever it hit, or is specific to teeth; the word used seems to be related to teeth ("dens" from the French "dents", "augeo" from the same root as "augment"), but without another example of the spell in action, this must be purest speculation.

Given the apparent effect of this spell, it is hard to imagine what Draco's intent would be in casting it; the effect would seem more likely to enrage the victim, rather than incapacitate him, although one supposes that the lisp resulting from having teeth overlapping one's chin could render verbal spell-casting ineffective.

Greater Picture
While the spell and its use here seem almost pointlessly trivial, this is a demonstration of the fact that spells, like physical projectiles, will be reflected by other spells. This proves to be a useful point at the climactic battle in the cemetery at the end of this book, and in battles in later books as well.