Do you find that your nose gets ticklish when you step out into the sun suddenly sometimes? If so, you just might have the Autosomal-dominant Compelling Helio-Ophthalmic Outburst syndrome (ACHOO!) - a phenomenon noted even by Aristotle in his Book of Problems, but thought obscure enough by scientists to bother giving it its own above syndrome name/acronym until 1978! More simply called the Photic Sneeze Reflex (PSR), it may result from a neural crosswiring in one's face - a current leakage between the optic (which controls pupil contraction due to sudden sunlight exposure) and trigeminal (senses nasal irritation and causes sneezing) nerves! [Hat-tip to Mark Hoofnagle of Denialism Blog]
Aristotle mused about why one sneezes more after looking at the sun in The Book of Problems: "Why does the heat of the sun provoke sneezing?" He surmised that the heat of the sun on the nose was probably responsible.
Some 2 ,000 years later, in the early 17th century, English philosopher Francis Bacon neatly refuted that idea by stepping into the sun with his eyes closed—the heat was still there, but the sneeze was not (a compact demonstration of the fledgling scientific method). Bacon's best guess was that the sun's light made the eyes water, and then that moisture ("braine humour," literally) seeped into and irritated the nose.
Humours aside, Bacon's moisture hypothesis seemed quite reasonable until our modern understanding of physiology made it clear that the sneeze happens too quickly after light exposure to be the result of the comparatively sluggish tear ducts. So neurology steps in: Most experts now agree that crossed wires in the brain are probably responsible for the photic sneeze reflex.
A sneeze is usually triggered by an irritation in the nose, which is sensed by the trigeminal nerve, a cranial nerve responsible for facial sensation and motor control. This nerve is in close proximity to the optic nerve, which senses, for example, a sudden flood of light entering the retina. As the optic nerve fires to signal the brain to constrict the pupils, the theory goes, some of the electrical signal is sensed by the trigeminal nerve and mistaken by the brain as an irritant in the nose. Hence, a sneeze.
Yet another example of our not-so-intelligently-designed but marvelously quirky evolved bodies, eh?
[Hat-tip to Mark Hoofnagle of Denialism Blog]