Why Onions Make You Cry — And How to Actually Stop It

Few things in the kitchen are as universally humbling as cutting an onion. You can be a composed, confident cook — someone who handles raw chillies without flinching and debones a chicken without consulting a YouTube tutorial — and still end up weeping over a chopping board like you've just watched a Pixar film. Onions have been doing this to us for at least 4,000 years. And yet, until surprisingly recently, we didn't fully understand why. A study published in October 2025 by researchers at Cornell University takes a fresh look at the problem — not from a chemistry angle, but from a physics one — and the findings have some genuinely useful implications for anyone who cooks.

The Chemistry Behind the Crying (A Quick Recap)

Before getting to the new physics research, it helps to understand what's actually happening at the molecular level when you slice into an onion.

The substance responsible for your tears is called syn-propanethial-S-oxide. It's a lachrymatory agent — a compound that stimulates the lachrymal glands in your eyes, triggering an involuntary flushing response. Your eyes aren't being dramatic. They're doing exactly what they're supposed to do when confronted with an irritant.

Here's the interesting part: onions don't actually contain propanethial-S-oxide. They contain the ingredients for it, kept safely separated in different parts of the cell. When your knife breaks open those cells, an enzyme called alliinase is released and immediately gets to work, converting stored compounds into 1-propenesulfenic acid. That intermediate chemical then reacts with a second enzyme — a lachrymatory factor synthase — to produce the actual tear-jerking molecule.

This two-step enzymatic process is essentially a chemical defence mechanism. The onion isn't trying to make you cry out of spite. It's trying to deter insects and other organisms from eating it. You just happen to be collateral damage.

What's striking is how recently we filled in the full picture: the second step in this reaction — the role of lachrymatory factor synthase — wasn't identified until 2002. For a vegetable humans have been cultivating since the Bronze Age, that feels like a long time to be missing a key piece of the puzzle.

Why the Physics of Onion Cutting Actually Matters

Understanding the chemistry tells us what the irritant is, but it doesn't tell us how it travels from the onion to your eyes. That distinction matters more than it might seem, because if you want to stop the crying, you need to intercept the molecule before it arrives — not after.

The Cornell team used a precisely calibrated cutting device (described, delightfully, as an onion guillotine), combined with high-speed cameras and electron microscopy, to observe exactly what happens at the moment a blade meets an onion. What they found is that the chemical is carried in microscopic liquid droplets, not as a free-floating gas, as many people assume.

These droplets are released in two phases: a sharp initial burst the moment the blade first punctures the onion's surface, followed by a slower, more sustained release as the blade travels through the flesh. That first burst is the one that gets you — it fires upward and outward with surprising force before you've had a chance to react.

Sharp Knives: Now Backed by Physics, Not Just Chef Wisdom

Every culinary school, every professional cook, and every confident home chef will tell you to keep your knives sharp. The usual reasons are about control and safety: a sharp blade requires less force, which means less chance of the knife slipping and less chance of injury. That advice is correct and worth following on its own merits.

But the Cornell research adds a new, physics-based reason to the list. A blunt blade doesn't cut cleanly — it compresses the onion's cell membranes before finally breaking through them. That compression stores up pressure, so when the blade does eventually puncture the surface, it releases a far larger burst of droplets than a sharp knife would. A sharp blade, by contrast, parts the cells more cleanly and with less stored energy, producing a smaller initial spray.

The same study found that cutting speed also matters. Faster cuts produce more droplets. A slow, deliberate stroke with a sharp knife generates the least amount of spray — which is, admittedly, slightly counterintuitive. Most people's instinct when cutting onions is to get it over with as quickly as possible. That instinct is making things worse.

The Fridge Hack Doesn't Work the Way You Think

Chilling your onions before cutting them is one of the most widely shared kitchen tips on the internet. The logic seems reasonable: cold temperatures slow chemical reactions, so a cold onion should produce fewer tears. It's the kind of advice that gets passed down through generations and repeated in listicles without much scrutiny.

The Cornell research scrutinised it. And the results were not flattering for the fridge hack.

Chilled onions actually released more droplets in the experiments, not fewer. The researchers' explanation is plausible: cold temperatures cause onion tissue to stiffen. Stiffer tissue behaves more like a compressed spring — it absorbs more of the blade's pressure before giving way, then releases that energy in a larger burst. The very mechanism that people hope will calm the onion down appears to make it worse.

This doesn't mean cold onions can't feel slightly more pleasant to handle for other reasons — they may release volatiles into the air more slowly at lower temperatures, which could marginally reduce irritation in some circumstances. But as a strategy for reducing the droplet spray that carries the lachrymatory compound directly toward your eyes, refrigerating your onions is not the solution.

What About the Bowl of Water Trick?

Another popular piece of advice is to place a damp cloth or a bowl of water near your chopping board when cutting onions. The reasoning here is actually chemically sound: propanethial-S-oxide is highly water-soluble, so if the droplets hit water before they reach your eyes, they should be neutralised.

The Cornell study didn't test this directly, which is a gap worth noting. But the high-speed camera footage offers an indirect answer that isn't encouraging. The droplets travel upward — toward the face of the person doing the cutting. Water sitting on the worktop beside the board is positioned to catch sideways splashes and secondary bounces, but it won't intercept that initial upward burst, which is the one doing most of the damage.

A follow-up study testing water interception at different heights and angles would be genuinely useful here. For now, the honest answer is: it might help a little, but probably not enough to rely on.

The Food Safety Angle You Probably Haven't Considered

The implications of this research extend beyond personal comfort at the chopping board. Droplets sprayed during vegetable cutting don't only carry chemical irritants — they can also carry foodborne pathogens. If a contaminated vegetable is being cut on a shared surface, those droplets can travel across the board, onto adjacent food, and onto nearby surfaces, spreading potential contamination in ways that standard food hygiene advice doesn't always account for.

This gives the sharp-knife-and-slow-cutting advice a food safety dimension that applies in every kitchen, not just when you're dealing with onions. Reducing droplet spray during cutting is a meaningful way to limit cross-contamination. It's a finding that food safety researchers and professional kitchen trainers may want to incorporate into their guidance.

So What Actually Works?

Based on both the established chemistry and the newer physics research, here's what the evidence actually supports:

Use a sharp knife. This is the single most impactful thing you can do. A clean cut through cell membranes produces a smaller initial burst of droplets than a blunt blade that compresses and then ruptures the tissue.

Cut slowly and deliberately. Faster cuts generate more spray. Take your time.

Skip the fridge. Cold onions may actually make things worse by stiffening the tissue.

Consider goggles — seriously. This sounds absurd until you learn that professional prep cooks, who cut onions in industrial quantities every single day, routinely wear them. If it works for professionals, it works for you.

Ventilate your workspace. While the Cornell research focused on droplets, some of the irritant does travel as a volatile compound. Good airflow won't eliminate the problem, but it reduces the concentration of irritants around your face.

The bowl of water and damp cloth methods remain unproven for the specific mechanism the new research identifies, though they're harmless enough to try.

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There's something quietly satisfying about the fact that a vegetable humans have cultivated for four millennia is still yielding new scientific insights. The chemistry of onion tears wasn't fully mapped until 2002. The physics of how those tears reach your eyes has only just been examined in rigorous detail. Science, like a good stock, takes time — and onions, apparently, still have secrets to give up.

Until the next breakthrough, keep your knives sharp, slow down your cuts, and maybe invest in a pair of goggles. Your eyes will thank you.

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Frequently Asked Questions

Why do onions make your eyes water?

When you cut an onion, you break open its cells, triggering a two-step enzymatic reaction that produces a compound called syn-propanethial-S-oxide. This lachrymatory agent irritates the lachrymal glands in your eyes, causing them to produce tears in an attempt to flush out the irritant. The compound is carried to your eyes in microscopic droplets released during cutting.

Does putting onions in the fridge stop them making you cry?

Contrary to popular belief, recent research from Cornell University suggests that chilling onions before cutting may actually make things worse. Cold temperatures stiffen onion tissue, causing it to absorb more pressure from the blade before breaking — resulting in a larger burst of droplets when it does. The fridge hack is not supported by the physics evidence.

Does a sharp knife really help with onion crying?

Yes — and now there's physics research to back it up. A sharp blade cuts cleanly through cell membranes with minimal compression, producing a smaller initial spray of droplets compared to a blunt blade, which compresses the tissue before breaking it. Cutting slowly with a sharp knife appears to be the most effective method for reducing droplet spray.

Is the bowl of water trick effective for stopping onion tears?

The chemistry behind the idea is sound — propanethial-S-oxide is water-soluble — but the physics of how droplets travel makes it less effective in practice. High-speed camera research shows that the initial droplet burst travels upward toward the cutter's face, meaning water placed on the worktop is too low to intercept it. It may help with secondary splashes, but it won't stop the first wave.

Are onion tears harmful to your eyes?

No. The tearing response is your eyes' natural defence mechanism working correctly. Propanethial-S-oxide is an irritant, not a toxin, and the effects are temporary. The tears your eyes produce are genuinely flushing out the compound. There's no lasting damage from regular onion cutting, though persistent exposure in professional kitchen settings is a good argument for wearing protective eyewear.