Tell Me Why

Have you been curious about something you observed and want to know why? Ask JM! This is the first of a new series where we answer readers' questions. So do write in to us at jantarmantar.why@gmail.com. Be sure to write your name, class and address so we can publish it along with your question.
I was watching my mother cut onions. Soon she began to sniff, and then so many tears were in her eyes that she could hardly see what she was cutting. Why did cutting onions make her cry?
Onions are mostly water, plus some vitamins and sugar compounds. They also contain compounds that include sulphur, a natural chemical found in many smelly substances, such as skunk spray and garlic. This is one way that plants defend themselves – producing substances that repel creatures who might eat them. Other plants have thorns or stinging leaves, or are made of special cells that make them hard to chew.
When the onion is intact, a group of compounds called cysteine sulphoxides are kept separate from an enzyme called alliinase. But when you slice, dice or crush the onion, the barrier separating the compounds and enzyme is broken. The two come together, setting off a reaction: The alliinase causes the cysteine sulphoxides to become sulphenic acid.
Sulphenic acids are not very stable so they must change into something else. In an onion, the sulphenic acid has two options. Option one is that it can spontaneously condense, a reaction within itself, and become an organosulphur compound. Organosulphur compounds are what give onions their strong smell and flavor. A similar reaction happens in garlic, which is why it also has such a pungent flavour.
But option two for the sulphenic acid is unique. Another enzyme, called lachrymatory factor synthase, that's been hiding out in the cell comes into play and rearranges sulphenic acid into something called the lachrymatory factor. This is a chemical that irritates the eyes. Why?
The lachrymatory factor (called syn-Propanethial-S-oxide) then rearrange into a chemical known as syn-propanethial-S-oxide. This is a volatile liquid, meaning it turns in to vapour very quickly. The gas diffuses through the air and, on contact with the eye, it stimulates sensory neurons creating a stinging, painful sensation. Whenever anything irritates the eye, the automatic reaction is for tears to be released from the tear glands. This helps to dilute and flush out the irritant. This is why you cry while cutting onions.
It turns out that onions are particularly good at absorbing sulphur, which they use for making several amino acids which are in turn used to make proteins. Both the organosulphur compounds that give onions their intense flavour and the tear-inducing lachrymatory factor have very likely evolved as defense mechanisms. That is, they were meant to prevent being eaten or destroyed by insects, animals or parasites. 
Certain varieties of onions, especially sweet onions, have less sulphur than others and are thus less likely to burn your eyes. You can also freeze or refrigerate onions before you chop them; this slows down the enzymes responsible for the series of unfortunate chemical events. Scientists are also trying to modify the onion so that it causes less irritation. But then it may not be as tasty! Better to put up with a few tears and enjoy the taste!