spaceSpace and Physics

Why Does Cutting Up An Onion Make You Cry?


Robin Andrews

Science & Policy Writer

Ah, First World Problems. Also, something tells me that this person has never cut an onion before... TheWorst/Shutterstock

Why is it that onions make you cry when you begin to mercilessly dissect them? It’s fairly unlikely that it’s because you’ve formed an emotional bond with it and you’re sad to see it chopped up into little pieces – so it must be down to some rather cretinous chemistry.

As explained with typical eloquence by YouTube team Reactions – as part of a video series produced by the American Chemical Society – onions belong to a group of vegetables that absorb high quantities of sulfur compounds from the soil. These are converted by the onion plant into other compounds that can readily transmogrify into a gas.


Murdering an onion splits open cells, which release enzymes and these gas-prone compounds – combining the two forms a cacophonously named gas, syn-propanethial s-oxide, which quickly reaches your eyes. Detecting this chemical irritant, your eyes send a signal to your central nervous system, which in turn causes your eyes to weep in an attempt to wash it out.

Unfortunately, it wouldn’t be a good idea to genetically modify onions to remove these pesky compounds: They’re actually partly responsible for making onions so tasty in the first place.

Why cutting onions making you cry. Reactions via YouTube


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