When an onion is chopped, certain substances called lachrymators are released, irritating the lacrimal glands and the nerves that surround the eyes. Methionine and cystine, two members of the amino acid family, make up these molecules.
These substances are released into the air when an onion is cut or diced. When this happens, a “enzyme” transforms the amino acids into chemicals that act as lachrymators. This type of sulfuric acid irritates the eyes‘ nearby nerves, causing tears to develop.

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