This is a question that I have also wondered about. Many times, I too have stepped outside into the bright outdoors, to be overcome by a fit of sneezing. Well, after doing a bit of research onto the topic, I was surprised to learn that I am afflicted with a genetic condition called the ACHOO syndrome, also known as the photic sneeze reflex.
The ACHOO syndrome has long been noted by both ancient and modern medicine, as well as by popular culture. In fact, it is the only syndrome on the NCBI databases (an online database which catalogs human diseases) which cites a Berenstain Bears storybook as a reference! (http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=100820). It is also thought to be inherited genetically, in a dominant fashion, which means that if either of your parents are affected, there is a 50% chance that you will be too.
So why do we sneeze when we look at the sun? Well, the short answer is that two reflexes in your brain are crossed: the "bright light" reflex, and the "dust in your nose" reflex.
If you are still reading, then I assume that you want the long answer! Well first of all, we must understand what a reflex is. A reflex is simply an automatic reaction (something you can't control) to any external stimuli, in this case, bright light.
When the eye senses bright light (any light, not just the sun!), signals are sent from the eye to the brain by specialized sensory nerve cells, or neurons. This signal is then processed by the brain, which then sends a signal back to the eye along "motor neurons," which stimulate muscles in the iris (the coloured part of your eye) causing it to contract. This results in the pupil (the black hole in the middle of the iris) getting smaller, thus allowing less light into the eye, so that you are not blinded in bright light! This reflex is called the pupillary light reflex.
But we are talking about another reflex here, namely, the sneeze! A sneeze is triggered by "tickling" of the membranes of the nose, by dust, or say, a feather. This stimulation then sends and receives a signal from the brain similar to the pupillary light reflex. In this case however, the signal from the brain tells the muscles of the nose, throat and chest to forcefully expel air through the nose and mouth, thus clearing the nose of any dust. This reflex may have been much more important earlier on in evolution, when our survival in the wild may have depended on our sense of smell!
So there you have it. If you're part of that 25% of the population that sneezes when we look at the sun, it's all because your pupillary light reflex and the sneeze reflex pathways just got a little crossed.
CurioCity would like to thank Dr. Sarah Attwell for her expertise and help in answering this question from Bernard- Grade 10 - Montreal, Quebec.