Onions make you cry because cutting them breaks open cells that normally keep certain chemicals separated.
Once those chemicals mix, a tear-triggering compound forms.
Cutting starts the reaction
A knife breaks onion cells.
Enzymes suddenly contact sulfur-containing compounds stored elsewhere inside the cells.
That reaction produces a volatile chemical known as the onion’s lachrymatory factor.
Because it is volatile, it travels through the air.
Your eyes detect an irritant
The compound reaches the moist surface of your eyes and irritates sensitive nerves.
Your tear glands respond by producing more fluid.
The tears are an attempt to dilute and wash the irritant away.
Why do some onions make you cry more?
Onion variety, growing conditions, sulfur content, and cutting technique can all affect the reaction.
A dull knife crushes more tissue.
A sharp knife damages fewer cells.
That can reduce how much irritating compound is released.
Do kitchen tricks help?
Some do.
Chilling the onion can slow chemical reactions.
Good ventilation can move irritating vapor away from your face.
Goggles work for the obvious reason: they physically block the chemical from reaching the eyes.
Why does cooking stop the crying?
Heat changes the enzymes involved.
Once the onion is cooking, it no longer produces the same irritating vapor in the same way.
This is another piece of kitchen chemistry, much like why apples turn brown after cutting, although the reactions are completely different.
You cut the onion.
The onion releases chemicals.
And somehow you are the one crying about it.
And that’s the short explanation.
Related explanations: Why do apples turn brown?, Why does bread rise?, and Why does salt make food taste better?