The onion makes you cry with a defence gas

A whole onion doesn't sting your eyes. The drama starts the instant you slice it: breaking its cells releases enzymes that had been kept apart from the rest of its compounds.

Within seconds, those enzymes turn some sulphur molecules into a light, volatile gas. The gas rises, reaches your eyes and attacks the nerve endings that detect irritants. Your body responds the only way it knows: flooding them with tears to wash the intruder away.

And because it's pure chemistry, you can outwit it. Chill the onion in the fridge before cutting and the enzymes will work far more slowly. Use a sharp knife to rupture fewer cells. Or cut it under the extractor fan. The onion doesn't hate you: it's just mounting a chemical defence that existed millions of years before our kitchens did.