Dinner has suddenly become emotional
Cutting an onion breaks its cells and starts chemical reactions that release an airborne irritant. When it reaches your eyes, they produce tears to help wash it away. The American Chemical Society explains this chain of events.
You were only making dinner. A moment later, you look as though the recipe has said something deeply upsetting. The onion has not hurt your feelings. It has set off a chemical reaction.
A tiny molecule with a very long name
The culprit is called syn-propanethial S-oxide. You can forget the spelling and keep the useful part: it is volatile, so it can travel through the air from a freshly cut onion to your eyes.
ACS describes the enzymes involved. These help create one compound and then turn it into the tear-provoking one. An enzyme helps a chemical reaction happen; you do not need to memorise every step to understand the result.
The knife starts the chain
The important change is the damage to the onion’s cells. Chopping opens them and allows the reactions to begin. The explanation is more specific than “onions smell strong”: an irritant is being formed and released.
Keep the sequence in mind: broken cells → chemical reactions → airborne irritant → tears. Those tears are the response at the end of the chain, rather than liquid coming out of the onion and somehow jumping into your eyes.
What happens if the onion is cooler?
ACS suggests refrigerating onions before chopping. Lowering the temperature slows the conversion and release of the compounds involved. It is a way to slow the chemistry, rather than a promise that no tears will appear.
Next time an onion interrupts dinner, the question is less “Why am I so emotional?” and more “What did cutting those cells allow to happen?” A kitchen can be a chemistry lesson with a chopping board.
For another everyday mystery, discover why thunder comes after lightning.