@Dolly da Gucci,
Onions contain amino acids (protein), vitamins and starch and this is what makes onions a good and tasty food. As part of its natural defense mechanisms, onions, when sliced release an enzyme that converts its amino acids sulfoxides into sulfenic acids. This acid can easily be converted to a gas by the action of another enzyme, lachrymatory factor synthase. The gas produced irritates your eye’s cornea, whose nerve endings will send a message to the brain and activate your lachrymal glands to produce tears in order to flush out the irritant.