Why do we shed tears while cutting an onion?

In Onions there will be Sulphur which produces Amino acid. When we cut an onion the element Sulphur in it splits and produces a liquid called Propanethial S-Oxide. This liquid got a peculiar property that it evaporates as soon as it is formed. This vapour enters into our eyes and converts into Sulphuric acid and Hydrogen sulphide liquid after mixing with water (wetness) in the eyes. This gives the eye a feeling of severe burning, and not only tears fall from the eyes but even water flow from the nose.

It is intersting to know that this sulphur compound only gives aroma when we cook the onions. To avoid tears coming out of the eyes while cutting the onions, onions are to be washed with cold water before cutting and they should be kept wet. Another way to avoid tears is by keeping the mouthopen while cutting the onions, so thet the gases coming out of the onions will enter directly into the mouth and they will not enter into the eyes. If one feel it is difficult to keep the mouth open for a long time better to keep a piece of bread between the lips. The bread piece will absorb the gases emerging from the onion and they will not enter into the eyes directly.