Onions make us cry not because they’re emotional, but because of the chemicals inside them.
When an onion is cut, its cells break and release an enzyme called alliinase. This enzyme reacts with sulfur compounds in the onion, producing a gas called syn-propanethial-S-oxide.
The gas rises into the air, and when it reaches our eyes, it reacts with the water in our tears, forming a mild acid that causes a burning sensation and makes our eyes water.
The fresher the onion, the stronger the reaction.
To reduce the effect, try cutting onions under running water or chill them in the fridge before slicing.
Cooked onions don’t make you cry because heat destroys the enzyme responsible for producing the gas.