I believe you have sliced onion before and were choked to tears by its unique smell, right?  

According to a university professor of the food science department, this is because onion contains a kind of sulfur-containing amino acid, e.g., methionine and cysteine. When an onion is sliced, its cells would be damaged, and its enzymes would decompose the sulfur-containing amino acid, eventually producing volatile sulfuric acid.

When this smell gets in touch with our nose or eyes, it will irritate the nerve endings of the cornea of our eyes, and it causes the lacrimal gland to produce tears to flush away the stimulus. 

What then are those little techniques that keep us from being in tears when we slice the onion?

Technique 1: The stimulus of sulfuric acid of the onion is mainly centralised in its root. Therefore, we should cut off the root before slicing the onion. If we were to keep the root, then we would have to cut it at the end! 

Technique 2: Soak the half-sliced onion in cold water for about 10 minutes before slicing it further, or you can slice it in the water. This is because the internal chemical substance of the onion is water soluble, and it can be washed away by soaking it in the water, to minimize the volatility of the stimulus. 

Technique 3: Put the onion in the fridge before slicing it as this could decrease the volatility of its stimulative smell.

Technique 4: The study of a Japanese researcher discovers that when one slices the onion into halves and put it in a microwave for 2-3 minutes, it helps to reduce the stimulation of the sulfuric acid.  

Finally, if your lacrimal gland or your nose is super sensitive, and the methods above are not workable for you, then you might have to use tissue or cotton to block your nostrils and wear a pair of swimming glasses or goggles when you slice the onion! Haha!