Somewhere between 18 and 35 percent of the population has this reflex, which is a genetic quirk—a dominant genetic trait, to be exact. It's been in our DNA a long, long time: the first reported case of this was recorded by philosopher Aristotle. In The Book of Problems, he wrote, "Why doth the heat of the sun provoke sneezing, and not the heat of the fire?" way back in 350 B.C.

Like so many strange things our bodies do, nobody knows why it happens, but we have some guesses. The most popular theory goes like this: the trigeminal nerve is the one responsible for making you sneeze when your nose is irritated. The optic nerve is responsible for constricting your pupils when you walk out into bright sunlight. These nerves are very close to each other, and it's possible that solar sneezing comes from an accidental crossing of the streams: your brain might be mistaking the optic nerve's signal that you've encountered bright sunlight as a signal from the trigeminal nerve saying there's something irritating your nose.