What makes you cry when cutting an onion?

Answer

Cutting breaks cells and enzymes make the gas, which irritates the eyes.

Context

An intact onion keeps its ingredients apart. Sulfur-containing precursor molecules sit in one compartment of the cell, and an enzyme called alliinase sits in another. Cutting ruptures the cells and lets them mix. Smithsonian Magazine describes the next steps: alliinase converts the amino acid-derived precursors into sulfenic acids, and a second enzyme, lachrymatory factor synthase, rapidly rearranges one of them into the volatile compound that irritates the eyes. The enzyme was identified in 2002.

The irritating compound is syn-propanethial-S-oxide. It evaporates, drifts to the eye and stimulates sensory nerves in the cornea, which triggers reflex tearing and a stinging feeling. The American Chemical Society notes that the compound is unstable and changes into other molecules when it is in water, which is how tears eventually wash it away. Since the reaction depends on enzyme speed, the amount of gas released varies with onion variety, growing conditions and how much tissue is damaged.

Researchers have tried to make onions that do not produce the irritant. Smithsonian reports that a New Zealand group removed the gene for the synthase in 2008 and that a Japanese team used ion bombardment in 2015, though neither product reached stores. Chilling onions is sometimes said to slow the release of fumes, and sharp knives are said to rupture fewer cells; the first is mentioned in Smithsonian's report, while the second is common kitchen advice not covered by the sources here. Another misconception is that the onion's smell and its tear gas are identical; they involve different molecules.