Chewing something (like bread) can keep you from crying when cutting onions.
When we cry, the tears do not actually come from our eyes. They come from the lachrymal glands, which are small glands which produce tears on irritation and are located next to the eyes.
The onions contain an unstable chemical known as syn-propanethial-S-oxide. When an onion is cut, this chemical is released into the air and it irritates the lachrymal glands, causing them to excrete tears.
To prevent tears brought by onions, you can chew something like bread, as continuous chewing forces you to breathe through your mouth. This disperses the irritant so that a significantly smaller amount reaches the lachrymal glands, preventing them from being irritated enough to release tears.
Here are some other ways to prevent tears while cutting onions:
1) Use a sharp knife.
The enzymes are released when the onion cells are crushed. A sharp knife simply slices the onion in a way that its cells are not crushed, releasing fewer enzymes.
2) Chill your onions in the freezer for 10-15 minutes before cutting them.
This reduces the amount of enzymes released in the air and does not affect the onion’s taste.
3) Cut the onion next to a strong draft from a fume hood, fan, or window.
This will carry the enzymes of onion away from your eyes.
4) Soak the onion in water.
This dilutes the enzymes but makes the onion a bit slippery, so you have to be careful while slicing the onion.
5) Wear swimming goggles.
If onions cause you more irritation, goggles can prevent the onion gases from getting into your eyes.
These methods can effectively prevent the irritation and tears caused by onions.
Reference : wikihow