Muggles' Guide to Harry Potter/Magic/Bubble-Head Charm

Overview
The Bubble-Head Charm makes a pocket of breathable air about the caster's head.

Extended Description
This charm has been seen used in two places so far.

In Harry Potter and the Goblet of Fire, Cedric and Fleur use the Bubble-Head Charm in the Second Task. This charm allows them to survive underwater, giving them breathing air, for the time necessary to carry out their task.

In Harry Potter and the Order of the Phoenix, after Dolores Umbridge is appointed Headmaster, discipline at the school falls apart. Because she is afraid that the teachers might act to undermine her, Umbridge basically removes any authority they have, which leaves her and Filch the impossible task of maintaining discipline in a school of possibly 680 adolescent witches and wizards. While teachers can maintain order in their individual classrooms, nobody will take responsibility for the hallways, which become littered with dungbombs, which Filch, being a Squib, is largely powerless to clean up. It becomes common practice for the students to use a Bubble-Head Charm on themselves between classes, because the hallways smell so bad.