Physicists have told you how to cut onions and not cry

Everything is much more... scientific! Scientists from Cornell University (USA) have discovered why onions cause tears - and more importantly, how to avoid it.

In a new study published on arXiv, a team of physicists, biologists and engineers decided to take the problem seriously: they built a special guillotine (don't worry, just for onions), used high-definition cameras and even looked into a microscope. And here's what they found out.

Details: Zixuan Wu et al, Droplet Outbursts from Onion Cutting, arXiv (2025). DOI: 10.48550/arxiv.2505.06016

💧 Where do the tears come from?

When we cut an onion, it releases a substance with a long name - syn-propanethial-S-oxide. It's what irritates our eyes and makes us cry. But how much of this substance is released depends on exactly how we cut the onion.

🔪 It's all about the knives

Scientists have found out: the sharper the knife and the slower you cut, the fewer tears you' ll produce. Why? Because a dull knife puts pressure on the onion, compressing its layers, and then those layers spring back - and literally "spill" onion juice into the air. And the faster you cut, the more juice in the air. And the more tears.

In the video, the scientists noticed that the onion juice droplets flying into the air are broken into small particles. Which means they "hang" in the air longer and irritate your eyes even more.

🧪 And how did they test this?

• They used a guillotine with different blades

• Covered the onion slices with paint to make it obvious

• Changed the speed and angle of the cut

• They analysed the droplets through an electron microscope

Bottom line: the duller the knife and the faster the movement, the more onion mist in the air. And that's a direct route to a crying chef.

✅ What to do to avoid crying?

• Use the sharpest knife possible

• Cut slowly and without sudden movements

• If possible, cool the onions before cutting (this method has long been known, and it also helps to reduce the release of juice)