why do we cry when we slice onions?!

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Because onions release a gas when cut near the root, so don't cut off the root first since that's where the gas is stored. Also you can put your onions in the fridge for an hour before cutting and you won't cry. =,)

As onions are sliced or eaten, cells are broken, allowing enzymes called alliinases to break down amino acid sulphoxides and generate sulphenic acids. A specific sulfenic acid, 1-propenesulfenic acid, formed when onions are cut, is rapidly rearranged by a second enzyme, called the lachrymatory factor synthase or LFS, giving syn-propanethial-S-oxide, a volatile gas known as the onion lachrymatory factor or LF.[21] The LF gas diffuses through the air and eventually reaches the eye, where it activates sensory neurons, creating a stinging sensation. Tear glands produce tears to dilute and flush out the irritant. Chemicals that exhibit such an effect on the eyes are known as lachrymatory agents.

Supplying ample water to the reaction while peeling onions prevents the gas from reaching the eyes. Eye irritation can, therefore, be avoided by cutting onions under running water or submerged in a basin of water.[22] Another way to reduce irritation is by chilling, or by not cutting off the root of the onion (or by doing it last), as the root of the onion has a higher concentration of enzymes. Using a sharp blade to chop onions will limit the cell damage and the release of enzymes that drive the irritation response. Chilling or freezing onions prevents the enzymes from activating, limiting the amount of gas generated. Eye irritation can also be avoided by having a fan blow the gas away from the eyes as the onion is being cut.

It is also possible to avoid eye irritation by wearing goggles or any eye protection that creates a seal around the eye. Contact lens wearers can experience less immediate irritation as a result of the slight protection afforded by the lenses themselves. It may[citation needed] also be that lens wearers are familiar with controlling reflexive actions of their eyes, such as blinking, with regard to irritation, as this is an ability useful when manipulating the lenses.

The amount of sulfenic acids and LF released and the irritation effect differs among Allium species. On January 31, 2008, the New Zealand Crop and Food institute created a strain of "no tears" onions by using gene-silencing biotechnology to prevent synthesis by the onions of the lachrymatory factor synthase enzyme.

http://en.wikipedia.org/wiki/Onion

When you cut an onion, you break cells, releasing their contents. Amino acid sulfoxides form sulfenic acids. Enzymes that were kept separate now are free to mix with the sulfenic acids to produce propanethiol S-oxide, a volatile sulfur compound that wafts upward toward your eyes. This gas reacts with the water in your tears to form sulfuric acid. The sulfuric acid burns, stimulating your eyes to release more tears to wash the irritant away.
This makes you cry very much and sometimes you have too step awayt ostop it from crying

When you cut an onion, you break cells, releasing their contents. Amino acid sulfoxides form sulfenic acids. Enzymes that were kept separate now are free to mix with the sulfenic acids to produce propanethiol S-oxide, a volatile sulfur compound that wafts upward toward your eyes. This gas reacts with the water in your tears to form sulfuric acid. The sulfuric acid burns, stimulating your eyes to release more tears to wash the irritant away.

http://chemistry.about.com/od/chemistryf…

Bob and the others have done a great job explaining, so I don't have much to add on the reason why, but I can add info on an interesting prevention/remedy:

I had a Brazilian exchange student many years ago and she told me that she puts water in her mouth when she cuts onions. I don't know if it really works - it may be a mumbo jumbo, old wives tale, but you might want to try it:)

I don't have problems with onions myself, but that might be because of my contact lenses and blinking technique:)

It's a very emotional experience. Personally I never eat any vegetables because I hate to think of the pain and suffering a carrot or potato must feel when being callously plucked from the earth. The squealing of cattle as their throats are slit, however, I find rather appealing.

The cells of onions are loaded with sulfuric acid and when you cut them the acid is released into the air.

Because, the tension, and scent of the onion is so strong, it makes your eyes water.

because there's some kind of gas that get out
when you slice the onions

i cook

because onions are bitches