Onions cause tearing when sliced due to a chemical reaction, and there are practical methods to prevent this phenomenon.

noli's read

When an onion is cut, its cells are ruptured, releasing enzymes (alliinases) that transform sulfur compounds into a lachrymatory gas (syn-propanethial-S-oxide). Upon contact with the moisture in the eye, this gas turns into mild sulfuric acid, irritating nerve endings and triggering tears. Biochemical research widely confirms this mechanism (Journal of Agricultural and Food Chemistry). To limit the effect, techniques rely on reducing gas diffusion: using a very sharp knife reduces cellular crushing, and refrigerating the onion before cutting slows the kinetics of the enzymatic reaction. While these methods are scientifically consistent, other popular hacks (such as keeping a piece of bread in one’s mouth or lighting a candle) lack rigorous evidence and are often anecdotal. Science explains the 'why' effectively, whereas effective 'how' methods are mostly limited to temperature control and the quality of the cut.

the sources

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