Why tears come while cuting a onion?
When an onion is cut certain(lachrymator) compounds are released causing the nerves around the eyes (lacrimal glands) to become irritated. As a result of this tears will come out. These compounds consist of methionine and cystine that are part of the amino acid family. Irritation to the tear glands causes it to secrete tears continuously without being influenced by the intent and emotion to cry. The chemical compounds released by the onions are allyl sulfide and they also exist in garlic and spring onions.
When an onion is sliced or diced, the onion's cells release these compounds into the air. When this occurs, "enzyme" works to alter the amino acids into lachrymator compounds. This form of sulfuric acid irritates the nerves around the eyes making them tear.