You look at the sun, you sneeze, and nobody knows why

Aristotle asked the question around 350 BC: why does the sun make us sneeze when the heat of a fire does not? His answer was that the sun warms the nose and makes it sweat.

Two thousand years later, Francis Bacon, an English philosopher with a habit of testing things, stepped into the sun with his eyes shut. He did not sneeze. So heat was not the cause: the cause was coming in through the eye.

In 1978 a group of geneticists finally gave it a name and an acronym: ACHOO, autosomal dominant compelling helio-ophthalmic outburst. Dominant means inheriting it from one parent is enough. It happens to between 18 and 35% of the population, and always the same way: they step out of a doorway, a tunnel or a cinema and let off two or three sneezes in a row.

The best-supported explanation today is that the optic nerve and the trigeminal nerve, which serves the nose, run so close together that the light signal leaks into the wrong circuit. We say "leaks" because nobody has shown where.

It is not a harmless curiosity: the United States Air Force has it documented as a risk for a pilot coming out of cloud. And it is hereditary: if you sneeze on stepping outside, half your children will too.