Anyone who has ever sliced an onion, knows the eye stinging feeling and watering that comes with it. 

A new study from researchers at Cornell University, published in the Proceedings of the National Academy of Sciences, has uncovered exactly how those eye-stinging onion chemicals blast into the air and revealed a surprisingly simple way to stop them. 

Onions have long been known to release an irritant called propanethial S-oxide, a sulphur-based compound that triggers your eyes’ tear reflex. But what no one really understood, until now, was how that chemical escapes from the onion and launches itself straight toward your face. 

Using high-speed cameras and computer modelling, the Cornell team watched in extreme slow motion what actually happens when a knife slices through an onion. 

They found a two stage tear cascade 

As your knife presses down, it creates pressure within the tightly packed onion cells. Each onion layer is trapped between two skins, a top and a bottom forming little pressurised chambers. 

When the knife finally breaks through the top layer, all that pressure releases in an instant shooting a fine mist of droplets into the air. 

Then comes stage two - slower streams of fluid form into droplets that continue to spray outward. 

The researchers dubbed this phenomenon the two-stage tear cascade a microscopic explosion that propels irritant droplets at astonishing speeds of 18 to 143 km per hour. 

The scientists discovered that your choice of knife, and how you use it makes a big difference. 

A sharp knife creates a cleaner, slower cut, releasing far fewer droplets. But a blunt blade, especially when used quickly, causes chaos smashing through onion layers, increasing pressure, and launching more mist into the air. 

So, the simple takeaway? Keep your knives sharp and slice slowly. 

Alternatively, coating the onion in oil before cutting can also help trap the mist, though that might be less convenient for everyday cooking. 

Beyond just helping us cook without crying, this research offers fascinating insights into fluid dynamics, food safety, and the everyday physics hiding in your kitchen. 

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