Sun sneezing, or the Photic Sneeze Reflex, is a phenomenon that causes some people to have a sneezing fit when they look at a bright light. Scientists call the effect an autosomal dominant compulsive helio-ophthalmic outburst (or ACHOO for short, gosh aren't they witty).
Whilst this sounds like something of an old wives tale, it is actually a real thing ... we just don't quite know why. One thing we do know, though, is that it's most likely genetic.
Sneezing is usually caused by irritation to the nasal cavity, so that looking at the sun should irritate your nose is a bit odd.
One possible explanation is that the eyes and the nose are closely linked via the fifth cranial, or trigeminal, nerve. This means that when this nerve is activated by bright light, the signal also kinda tickles the nose, tricking it into thinking there is something irritating it and causing the sneeze.
Whilst the effect seems pretty harmless, or even useful if you have one of those sneezes that won't quite come, it can actually be pretty dangerous. A photic sneeze can often be triggered by suddenly emerging from a tunnel whilst driving and it has even been recognised as a real problem for both commercial and military pilots.
But even if we're not quite sure what causes it, we can still prevent the sun sneeze. The best remedy? A sweet pair of shades.