KnowHOW team explains: Both onions and garlic contain derivatives of sulphur-containing amino acids. When an onion is sliced, one of these compounds, S-1-propenyl cysteine-sulphoxide, is decomposed to a volatile chemical which is the irritant. Upon contact with water ' the surface of the eyes is always wet ' the irritant gets converted into propanolol, sulphuric acid and hydrogen sulphide. The eyes produce tears in an attempt to dilute and wash away the acid.
Various traditional remedies advise keeping a slice of lemon, bread or a sugar cube between the lips. The idea behind them seems to be to induce breathing through the mouth instead of the nose, which creates a current of air flowing up to the eyes.
Somewhat better methods are to stand well back from the chopping board rather than over it, or to cut the onions under water. If these seem cumbersome, wash the onion before slicing, and keep wetting it while you go on slicing.
The question was sent by Amitava Bhadra from Ghatakpukur, South 24-Parganas.