Nose-picking
Nose-picking, or rhinotillexis (Greek, rhino "nose" + tillexis "habit of picking"), is the act of extracting mucus and/or foreign bodies from the nose with a finger or other object. This is an extremely common habit, with some surveys indicating that it is almost universal, with people picking their nose an average of about four times a day.
The mucus membranes constantly produce wet mucus that is exposed to dry inhaled air. Once dried, the mucus typically causes a sensation of irritation that leads to the compulsion to dislodge the itch via rhinotillexis. However, extreme nose picking resulting in severe nasal trauma is termed rhinotillexomania (etymology: Greek, rhino "nose" + tillexis "habit of picking" + mania), it may be caused by Obsessive Compulsive Disorder or Body Dysmorphic Disorder.
Although a very common habit, it is a mildly taboo subject in most cultures. While nose picking appears to be an almost universal practice amongst humans, the observation of the activity in another commonly provokes the basic emotion of disgust. The earliest record of nose picking comes from ancient Egypt, ca. 1330 BC, where a papyrus scroll found by the eminent archaeologist Dr. Wilbur Leakey details the financial payment of three heads of cattle, and food and lodging, to Tutankhamen's personal nose picker. A 1995 study into nose picking requesting information from 1000 randomly selected adults gathered 254 respondants. It defined nose picking as "the insertion of a finger (or other object) into the nose with the intention of removing dried nasal secretion." Of those whom responded, 91% were current nose pickers (but only 75% of these believed everyone did it) and 2 people spent between 15 to 30 minutes and 1 to >2 hours a day picking their nose.
Nose-picking may carry a number of medical risks, thus most doctors recommend against it and encourage the use of a tissue instead. Risks include nasal infections, occasional nosebleeds (in 25% of those who pick their nose) and in rare cases perforation of the nasal septum  (self-induced ethmoidectomy). Nose picking, however, should not affect the sense of smell, as the nasal cavity where the olfactory nerves are located are too high up to reach. Also, due to the special nature of the blood supply to the nose and surrounding area, it is possible for retrograde infections from the nasal area to spread to the brain, although this scenario is unlikely to arise from normal rhinotillexisis. For this reason, the area from the corners of the mouth to the bridge of the nose, including the nose and maxilla, is known to doctors as the "danger triangle of the face." A minority of doctors claim that nose-picking, combined with nasal mucus eating, may benefit the immune system.
Children's literature often makes reference to it, to amuse readers (for example Jacques Charpentreau's poem "De l'Education!" and Gordon Korman's Nose Pickers series). John Allen Paulos's imaginary novel, Rucker: a life fractal has a section where "proboscis probing is discussed at length."
A popular saying holds that "You can pick your friends and you can pick your nose, but you can't pick your friends' noses."  This phrase became the title of a talk show parody on Saturday Night Live on March 15, 1986; in that sketch, some members of a television panel attempted to have a discussion while their colleagues made efforts to pick each others' noses, thus earning a rebuke from the host for violating the rules of the show.