Some sneezes make more sense than others.
If a speck of dust gets into your nose, you sneeze to get it out. If you smell the pollen of something you're allergic to, your sneeze is trying to get rid of that pollen.
Roughly a quarter of us however, have an inherited condition known as "photic sneezing." If you have this, it means that you also sneeze when you see a bright light. How can we make sense of a sneeze like that?
Francis Bacon thought he had the condition figured out in 1635, "looking against the sun doth induce sneezing," he observed. "The cause is not the heating of the nostrils," he reasoned, "but the drawing down of the moisture of the brain."*
Nowadays, we suspect that photic sneezing is due to the very close link between the protective reflexes of our eyes and our nose.
You may have noticed that whenever you sneeze, your eyes are likely to close and water. This protective reflex of the eye is triggered automatically by our nose's protective reflex, the sneeze, and for good reason. Chances are, the dust or other irritant that has gotten into your nose might also irritate your eye.
Photic sneezing occurs when the signal gets crossed, and the reflexes are linked in the other direction. A sudden bright light will make your eyes squint to protect themselves, but a photic sneezer will also sneeze. It's simply a case of your protective reflexes erring on the side of caution.
*The Bacon quote is edited for spelling and punctuation. It comes from Sylva Sylvarum.