when we cut onion tears come out from eyes,why?!

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It is not the strong odour of the onion that makes us cry, but the gas that the onion releases when cut. The onion itself contains oil, which contains sulfur, an irritant to both our noses and to our eyes. Cutting an onion arouses a gas contained within the onion, propanethiol S-oxide, which then couples with the enzymes in the onion to emit a passive sulfur compound. When this upwardly mobile gas encounters the water produced by the tear ducts in our eyelids, it produces sulfuric acid. In response to the caustic acid, our eyes automatically blink, and produce tears which irrigate the eye, and which flush out the sulfuric acid. I hope this helps! :)

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Girlygirl101

Me.

It is not the strong odour of the onion that makes us cry, but the gas that the onion releases when cut. The onion itself contains oil, which contains sulfur, an irritant to both our noses and to our eyes. Cutting an onion arouses a gas contained within the onion, propanethiol S-oxide, which then couples with the enzymes in the onion to emit a passive sulfur compound. When this upwardly mobile gas encounters the water produced by the tear ducts in our eyelids, it produces sulfuric acid. In response to the caustic acid, our eyes automatically blink, and produce tears which irrigate the eye, and which flush out the sulfuric acid.

Chopping onions unleashes a “chemical defense that onion plants have to protect themselves against insects and microbes

When onion cells rupture – whether through an insect’s nibble or a knife’s cut – allinase and ACSO mix together and react, producing another set of sulfur compounds called thiosulfinates. In addition to giving onions their familiar taste and odor, thiosulfinates repel pests that attack onion bulbs underground.

They aren’t the chemicals that cause tears to well, however. Before the thiosulfinates are produced, the reaction of allinase and ACSO releases a volatile chemical that wafts into the air and reacts with the water in our eyes. “Basically it produces something like sulfuric acid in your eye,” says Goldman.
The mild acid irritates the eye’s nerve cells, stimulating tears that help wash it away. Chilling onions before chopping can reduce the crying, says Goldman, because “this is an enzyme-mediated process, and low temperatures slow enzymes down.”
Sweet onion varieties, like Vidalia, grown in the low-sulfur soils of Georgia and Texas also show less of this activity and are less pungent

When you cut an onion, it releases a gas called, ready for this, Propanethiol S-oxide. When mixed with certain enzymes in the onion, it creates a sulfur gas. These gases then get to your eyes and create a mild acid which irritates the eyes.Normally, your body would signal you to close your eyes. This, of course, is not a good idea if you are cutting an onion. The next best thing your eyes can do is to make tears to protect the eyes. Rubbing your eyes is a bad idea, since your hands are likely full of the tear making onion juice. So how do you keep from tearing up? There are many theories out there. I've heard some have had luck burning a candle near the chopping site

http://www.sciencebob.com/questions/q-on…