Cutting an onion releases an enzyme called lachrymatory-factor synthase. This starts the process that leads to tears. This enzyme then reacts with amino acids of the onion and the amino acids are converted to sulfenic acids. The sulfenic acids spontaneously rearrange to form syn-propanethial-S-oxide, which is released into the air. When this chemical reaches the eyes, it triggers the tears by contacting nerve fibers on the cornea that activate the tear glands. Now you are crying.
Scientists have tried to make a “noncrying” onion but it seems that the crying enzymes are also responsible for the zesty onion flavor. But there may be some hope on the way. The group of Japanese plant biochemists that only recently discovered lachrymatory-factor synthase, the crying enzyme, believe that “it might be possible to develop a nonlachrymatory onion by suppressing the lachrymatory-factor-synthase gene while increasing the yield of thiosulphinate.” Sounds delicious!
In the meantime there are several solutions to try to avoid the problem of onion-induced tears. Heating onions before chopping, cutting under a steady stream of water, or wearing goggles.
The most reliable: ordering takeout. 
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"Why do I cry when I cut onions?" was posted on Wednesday, April 2nd, 2008 at 11:04 pm.