Show me a someone who say they cannot cry and I will make them.
That line belongs to an onion. You might laugh, roll your eyes, or quietly admire the wit, but there's a real point buried in the joke. Anyone who has helped out in the kitchen knows the feeling: the steady stream of tears, the running nose, the sudden urge to step away from the chopping board. These aren't tears of emotion, and they're not really the tears of heartbreak either. They're the tears that come when you chop, slice, or dice a vegetable from the family allium — better known as the humble onion.
Why do onions make us cry?
It's not because they're humorless. It's chemistry, and it starts underground. Onions grow in the soil, and while the green shoots rise above the ground, the round bulb we use in the kitchen stays buried beneath it. Because onions live under the soil, they absorb the minerals and nutrients they need to grow — and here's the catch: they're especially good at absorbing sulfur.
The gas that does the damage
Inside the onion, there are amino acid sulfoxides. Think about what happens when you get a cut on your skin — blood flows out, and everything mixes together. Something similar happens when an onion is sliced. The tear in the cells allows the amino acid sulfoxides to mix with certain enzymes, and that reaction produces a chemical called Syn-Propanethial-S-Oxide. This compound is volatile, which means it takes the form of a gas even at normal temperatures.
What happens when it reaches your eyes
When this gas drifts upward and reaches the protective layer over your eyeball, it reacts to form a very, very mild form of sulphuric acid. Before you panic at the thought of acid in your eye, rest assured — it's extremely mild. But it's enough that your eyes, as part of their self-defence, produce a rush of tears to wash the irritant away. That tearful reaction is the whole reason chopping an onion makes you cry.
How to dodge the tears
You might be wondering whether there's any way to escape the waterworks. There isn't a perfect fix, but there are a few tricks that can cut down the tears considerably.
- Chill the onion first. Refrigerating onions for a while before cutting slows down the initial reaction, which means less syn-propanethial-s-oxide forms in the first place.
- Soak them in water. Letting the onion sit in water for a bit before chopping can help dissolve some of those sulfur compounds before they ever reach your eyes.
- Keep the gas away from your eyes. Anything that stops that gas from drifting upward — good ventilation, a sharp knife, working under running water — can make a real difference.
The sharpness of the knife matters more than you'd think. A clean, quick cut damages fewer cells than a dull one that crushes the onion as it goes, which means less of that irritating gas gets released. Pair that with a little patience to chill the onion first, and you can often get through the whole chopping board with barely a tear.
So the next time your eyes start to water at the cutting board, remember: it's not you being emotional, and it's not the onion being dramatic. It's simply a little chemistry working exactly as designed. The crying is evidence of a clever defence mechanism, and the tricks to avoid it come from understanding that mechanism. Until next time, keep the curiosity going.