why do we sneeze when we look at the sun

The reason some people sneeze when they look at the sun is due to a reflex called the photic sneeze reflex, also known as "ACHOO syndrome" (autosomal dominant compelling helio-ophthalmic outburst syndrome). It is a genetic trait where bright light, such as sunlight, triggers an exaggerated response involving the trigeminal nerve. When bright light hits the eyes, the pupils constrict and this causing crossed wiring between the trigeminal nerve pathways, leading to a sneeze. This reflex commonly occurs when moving from darkness into bright light and affects about 15% to 30% of people. The sneezing is essentially a naso-expulsive reflex similar to a cough but triggered by light intensity changes rather than nasal irritation.