In brief: To finely chop an onion, cut it in half through the root, leave the root end intact, make vertical cuts up to it, then chop across. Tears are caused by syn-propanethial-S-oxide, which forms when a knife damages the cells. A 2025 Cornell University study measured that a dull blade flings out up to 40× more droplets than a sharp one, and fast chopping releases up to 4× more than a slow cut. So an sharp knife and a steady pace help most.
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Why onions make your eyes sting
A whole, uncut onion barely smells. It only starts to sting when a knife damages its cells. At that point, the enzyme alliinase comes into contact with sulfur compounds in the onion, producing an acid that another enzyme converts into syn-propanethial-S-oxide. This volatile substance enters the air, irritates the nerve endings in the cornea, and the eye defends itself with tears.
The other enzyme, lachrymatory factor synthase, was described by Japanese scientists from House Foods in 2002 in the journal Nature. Until then, it was thought that the tear-inducing substance formed spontaneously.
What Cornell researchers found
The chemistry of the tear-inducing substance has been known for a long time. A team from Cornell University in the US investigated how it gets from the cutting board to the eyes. The study was published in the scientific journal PNAS in October 2025. The researchers cut onions with blades of varying sharpness and at different speeds, and filmed the cuts with a high-speed camera.
It turned out that each onion layer acts like a small pressure cushion. The blade first bends the layer’s firm membrane and compresses the cells beneath it. When it finally cuts through the membrane, the built-up pressure sprays out a fine mist of droplets. According to Cornell Chronicle, they fly at speeds of 5 to 40 meters per second and reach up to about two-thirds of a meter high - roughly the face height of someone standing at the counter.
- A dull blade flung out up to 40× more droplets than a sharp one. Before cutting through the membrane, it compresses more cells beneath it.
- Fast chopping released up to 4× more droplets than a slow, smooth cut.
- A chilled onion sprayed out more droplets than one at room temperature. The authors themselves point out that this finding needs further verification.
According to videos measured by scientists, professional chefs move the blade at no more than about one meter per second. The droplets are therefore many times faster than the knife, so you can’t dodge them. You can only limit how many are produced.
Myths and facts about tears
What you’ll need
- A sharp kitchen knife with a blade about 20 cm long. A shorter blade gets lost in a large onion, while a longer one is unnecessarily unwieldy for small tasks.
- A stable cutting board. If it slides around on the counter, place a damp cloth underneath.
- An onion. The method is the same for yellow, red, and white onions.
How to finely chop an onion, step by step
- Cut off the pointed top of the onion. Leave the root end intact; it holds the layers together until you finish chopping.
- Stand the onion on its cut end and halve it vertically through the root.
- Peel both halves, removing the first tough layer as well.
- Place one half cut-side down, with the root pointing away from the hand holding the knife. Use the knife tip to make vertical cuts from the root toward the cut end, without cutting through the root. The spacing determines the size of the pieces: 2–3 mm for a fine chop, 5–10 mm for dicing.
- For a large onion, make one or two horizontal cuts parallel to the cutting board, again stopping short of the root. Skip them for a small onion; its layers will separate it into dice on their own.
- Chop the onion crosswise, from the cut end toward the root. Keep the fingers of your other hand curled into a claw and move them back as the onion gets smaller.
- Lay the last piece near the root flat and finish chopping it. Discard the root.
With a little practice, the whole thing takes less than a minute. But accuracy comes before speed, and a slower pace also saves your eyes, according to Cornell.
How to hold the knife and onion
The hand holding the onion forms a claw: tuck your fingertips under and rest the side of the blade against your knuckles. Guide the blade in a smooth forward and downward motion with your knife hand. Don't saw or chop from above; both squeeze the onion more.
Also watch the knuckles of the hand holding the knife. When the blade reaches the cutting board, there's little room for them with taller blades, and they tap against the board. That's why the knife's shape and handle position matter too. The article Why the SARACEN® knife's handle is in a different place from other knives explains this in more detail.
What to use to cut onions
SARACEN® is a kitchen knife with a saber-shaped blade made of 67-layer Damascus steel. Its handle and center of gravity are shifted toward the middle of the knife, above the cutting edge. This directs the pressure of your hand perpendicular to the blade rather than at an angle, and your knuckles won't hit the cutting board as you chop.
For onions, SARACEN® Original 220 mm is a good choice: it's compact and light for fine work with vegetables. If you prefer a pinch grip, choose the lightweight SARACEN® Eclipse 220 mm. SARACEN® PRO has a tall blade, so even the 175 mm length is enough for onions, and you can scoop up the chopped onion with the blade and transfer it straight to the pot.
Other ways to cut onions
Strips cut along the grain hold their shape better than half-moons during long sautéing because the knife cuts through fewer cell walls. That's why onions for caramelizing are cut this way.
How to tell if a knife is dull
Onion is a good test of sharpness. A sharp knife passes through the skin without hesitation, and sliced onion barely releases any juice on the cutting board. A dull knife first slips on the skin, then breaks through it, and the onion releases juice beneath it. If this happens to you, it's time to sharpen your knife.
SARACEN knives have a 20° edge angle. The article Blade Sharpening Angle explains why this angle was chosen. A double-sided SARACEN® 1000/6000 whetstone is all you need for home maintenance. Find a video guide on the Knife Sharpening page.
Frequently asked questions
Why do I cry when cutting onions?
Because the knife damages the onion’s cells, producing a volatile substance called syn-propanethial-S-oxide. It reaches the eyes, irritates the nerve endings in the cornea, and the eyes respond by tearing up.
How do I cut an onion without it stinging my eyes?
Cut slowly with a sharp knife. According to a 2025 Cornell University study, a dull blade ejects up to 40 times more droplets than a sharp one, and rapid chopping up to 4 times more than a slow cut. Turning on the range hood or wearing glasses can also help.
Does putting an onion in the fridge beforehand help?
It’s not certain. Cold slows the evaporation of the tear-inducing substance, but in a Cornell University measurement, a chilled onion sprayed more droplets than an onion at room temperature. The authors themselves point out that this result needs further verification.
How do I finely chop an onion?
Halve the onion through the root, peel it, and place one half cut-side down. Make vertical cuts toward the root, but don’t cut through it, then slice across the onion. The root end holds the layers together until you finish cutting.
Do I need to make horizontal cuts when cutting an onion?
Not for small or medium onions; their layers divide them into cubes on their own. One or two horizontal cuts are worthwhile only for a large onion, where the inner layers are wide.
Why does my onion fall apart when I cut it?
Usually because you cut off the root end. It holds the layers together, so leave it on the onion until the final cut.
What’s the best knife for cutting onions?
A sharp knife with a blade around 20 cm long, ideal for slicing with a pulling motion. Good options include the SARACEN Original 220 mm, the lightweight SARACEN Eclipse 220 mm, or the SARACEN PRO 175 mm with a tall blade for scooping.
The two things that save the most tears cost nothing: a sharp blade and a slower pace. Researchers at Cornell measured both with a high-speed camera, so this time it’s not just an old wives’ tale.