Which vegetable makes many people cry when cutting it?

The onion, and cutting it triggers a small chemical weapon. Intact onion cells keep an enzyme and a sulphur compound in separate compartments; slicing breaks the walls and lets them mix, producing a volatile gas that drifts up, reacts with the water in your eyes to make a weak sulphuric acid, and gets flushed out with tears.

The plant is not being spiteful about you specifically. The system evolved against insects and grazing animals, and it only fires when the tissue is damaged, which is why a whole onion in a bowl does nothing at all. Chilling the onion slows the reaction, a sharp knife ruptures fewer cells than a blunt one, and cutting under running water carries the gas away - all the folk remedies work, for the same reason.

Tearless onions now exist. Researchers in New Zealand and Japan silenced the enzyme responsible, and the interesting result was that the flavour changed as well - the same sulphur pathway that produces the irritant produces much of what an onion tastes of. Whether that is a fair trade is a matter of opinion, which is roughly where the commercial versions have landed.