Why do you sneeze when the sun gets in your eyes?

Answer 1

It's called the photic sneeze reflex. It's a defined entity and about 20% of the population have this, when you look at a bright light after coming out of a dark room and you have this irresistible urge to start sneezing. This has been investigated by the US military because they were a bit worried about their fighter pilots flying towards the sun and going into a sneezing fit. They don't know exactly what's happening at the nerve level, but what we think is happening is that there's a bit of cross-wiring going on. When you look at the sun, your pupil closes up and it gets much much smaller in order to stop as much light getting into your eye. But sometimes this can get a bit muddled up with the bit of the brain that thinks that your nose must be irritating. This triggers a sneeze instead. In the old days, people used to think that when you look at the sun, it made your eyes water and the tears trickled down your nose, making you sneeze. They did some experiments to test this and found that the tears would trickle too slowly to make you sneeze, so we think at the moment that it must be cross-wiring.

Answer 2

I think it must be that the intense light makes your eyes water, and some of those tears go down into the back of your nose through the little drainage tubes at the corners of your eyes. That extra liquid would irritate your nose, causing the sneezing reflex.