The Reason for Tear while Cutting Onions
Onions contain a sulphur-based oil. When cut, the cells that contain the sulphur compounds are broken and the compounds are converted into sulfenic acid, which in turn produces a highly volatile chemical called synpropanethial-S-oxide, which then rises into the air.
When the synpropanethial-S-oxide from onions comes into contact with our eyes, it reacts with the moisture in our eyelids and results in the production of sulphuric acid, which irritates the eyes and stimulates the lachrymal glands. When stimulated, these glands release tears to flush out the acid and protect the eye.