Photic Sneeze Reflex

because some people sneeze at the sun. It really does happen.

Wikipedia entry for Photic Sneeze Reflex here.

Photic sneeze reflex is a genetic autosomal dominant trait, which causes sneezing when exposed suddenly to bright light, possibly many times consecutively. It is also referred to as photic sneeze response, sun sneezing, photogenic sneezing, the photosternutatory reflex, being photo sensitive, ACHOO syndrome, and "Achooism," with its related backronym Autosomal dominant Compelling Helio-Ophthalmic Outburst syndrome. The condition affects 17 to 35 percent of humans.

The first mention of the phenomenon is probably in the later work attributed to Aristotle (Problems, book XXXIII).