Why Onions Make You Cry—and What Physics Has to Say About It

Cutting onions can often feel like an emotional rollercoaster. As soon as you slice through that delicate layer, tears start streaming down your face, turning what should be a simple cooking task into a sob-fest. But fear not, onion lovers! The culinary world is full of creative solutions to this problem. People have tried everything from chilling onions beforehand, to cutting them underwater, to wearing goggles, or even chewing bread to minimize tears. But what if the answer lies not in these kitchen hacks, but in the fundamentals of physics?

A team of physicists at Cornell University has taken a scientific approach to this age-old issue. Their recent (and not yet peer-reviewed) preprint study dives into the mechanics behind onion-induced tears. Using an experimental setup that included a miniature guillotine and some cleverly painted onions, they found that the sharpness of your knife and the speed at which you cut are the biggest factors influencing how badly onions make you cry.

The Onion’s Tear-Inducing Secret

To understand why onions have this effect, it helps to know a little about their biology. Onions are rich in sulfur-containing compounds. When their cell walls are broken—by cutting, chopping, or smashing—a series of chemical reactions are triggered. One compound in particular, syn-propanethial-S-oxide, is responsible for triggering your tear ducts. When released into the air, this compound forms a vapor that drifts upward and irritates your eyes. Your body responds by producing tears in an attempt to flush the irritant away.

This compound, essentially a form of natural defense, has been tormenting cooks for centuries. Despite thousands of years of culinary use—onions date back at least to 3000 BCE and were even placed in Egyptian tombs for their symbolic representation of eternity—the issue of eye irritation remains unresolved.

What makes this study different is the way the researchers chose to tackle the problem: not by altering the onion, but by closely examining the mechanics of how we slice it.

A chilled onion prepared for cutting to prevent tears.

A Tiny Guillotine and the Science of Slicing

The Cornell researchers designed an ingenious experiment. They built a small guillotine capable of holding various steel blades and used it to cut onion quarters. To track how the onion deformed when struck by the blade, they first coated the onion pieces in black spray paint. This wasn’t for dramatic effect—although it may have added some flair to the lab—but rather to provide contrast for high-speed cameras to better analyze how the onion responded to different blades.

Using an electron microscope, they precisely measured the width of each blade tip, ranging from ultra-fine edges just 5 micrometers thick to much duller tips up to 200 micrometers wide. The guillotine sliced onions at varying speeds—from as slow as 1.3 feet per second (0.4 meters per second) to as fast as 6.5 feet per second (2 meters per second).

The researchers then monitored the particles released during each cut. Their findings revealed a clear pattern: sharper blades released significantly fewer aerosol particles, and slower cutting speeds resulted in fewer airborne droplets compared to faster slicing.

A sharp kitchen knife ready for efficient chopping.

Why Dull Knives Are the Real Culprit

What they discovered about dull blades was particularly striking. A dull blade doesn’t slice cleanly through an onion. Instead, it crushes and compresses the vegetable’s skin and flesh before finally breaking through. This delay causes elastic energy to build up within the onion’s tissues, much like a compressed spring. When the blade finally cuts through, all that stored energy is released suddenly, sending droplets of onion juice flying—sometimes at speeds reaching 141 feet per second.

These tiny droplets contain the same eye-irritating compound, and their high velocity allows them to travel farther and spread more widely in the air. Even worse, the turbulence caused by this forceful release breaks the droplets into smaller fragments, making it easier for them to diffuse throughout your kitchen—and into your eyes.

In fact, dull knives were shown to produce up to 40 times more aerosol particles than sharp ones. That’s a staggering difference, and it underscores how crucial knife maintenance is, not just for safety and culinary performance, but also for your comfort while cooking.

Faster Isn’t Always Better

Speed was also a key factor. Faster slicing created more turbulence, which increased the number of particles released. The faster the cut, the more chaotic the fluid dynamics of the process became. In their tests, slicing faster resulted in up to four times more aerosol particles than slower cuts.

So, while speed might seem efficient, it actually makes things worse when it comes to tear-inducing onion vapors. A quick chop may save you a few seconds, but you'll pay for it in eye irritation.

Kitchen Implications

The implications of this research are refreshingly practical. If you want to minimize onion tears, the solution is simple: use a very sharp knife and cut slowly and deliberately. These two factors—blade sharpness and cutting speed—have a far greater effect on tear production than any folk remedy involving bread or goggles.

It’s also a reminder of how much knife maintenance matters in the kitchen. A sharp blade isn’t just about aesthetics or cutting performance—it’s about minimizing unnecessary trauma to both your ingredients and yourself. Sharpening your knife regularly or investing in a higher-quality blade could significantly reduce your tearful experiences in the kitchen.

A Handy Tool to Reduce the Tears 🥲 If chopping onions always brings on the waterworks, you don’t have to suffer through it! One simple way to cut down on the tears is by using a vegetable chopper. Instead of standing directly over the onion while slicing, this tool lets you quickly press the onion through sharp blades into uniform pieces. Less direct contact with onion fumes means less crying—and it also saves time in the kitchen. These choppers are easy to find online or in stores like Walmart, Target, or eBay, making them a worthwhile addition to your kitchen if onions are a regular part of your cooking.

Of Science and Saucery

This study is still in preprint form, which means it hasn’t yet undergone the rigorous scrutiny of peer review. Nonetheless, the data appears sound and the methodology thorough. The researchers combined high-speed imaging, particle analysis, and careful experimental control to uncover how tiny changes in cutting conditions dramatically influence the behavior of onion vapors.

It’s also a fun example of how physics and everyday life intersect. The project took a humble culinary annoyance and applied serious scientific rigor to it, using precise tools and instruments to understand something that affects millions of people on a daily basis.

And while the study doesn't touch on how to best use those finely sliced onion bits—be it in soups, stir-fries, or caramelized for a burger topping—it certainly makes the process of getting there more comfortable.

Conclusion

In the end, the takeaway is as clear as a freshly sliced Vidalia: if you want to avoid crying while chopping onions, don’t look to gimmicks—look to science. Keep your knife sharp. Take your time. And maybe leave the bread-chewing for the sandwich you'll make afterward.

What this study offers is a practical, evidence-based guide to making kitchen life a little more tear-free, all thanks to physics, a tiny guillotine, and some brave black-painted onions.

How Do You Fight Onion Tears?

• 0%Use a super sharp knife
• 0%Chill or freeze the onion first
• 0%Wear goggles while chopping
• 0%Cut onions under running water

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