Why Onions Make You Cry, and How to Stop It

An onion sitting whole on a counter is completely harmless. Cut it and within fifteen seconds your eyes sting. Nothing was waiting inside ready to attack: the irritant did not exist until the knife went in, and it took two separate enzymes to build it. That is the core of the why onions make you cry science, and the second enzyme was only identified in 2002, which means most reference books written before then are wrong. Better still, the kitchen folklore around it is testable in forty minutes, and three of the five famous remedies genuinely work.

Key Takeaways

• You will test five anti-crying remedies on the same onion variety and rate the effect, three trials each, in randomised order.
• The concept is enzyme chemistry: cutting releases alliinase and lachrymatory factor synthase, which together build a volatile irritant from a sulfur-containing precursor.
• Good for ages 7 to 13, with an adult handling the knife for the younger end.
• The thing that spoils the result: doing the trials in a fixed order. Expectation changes how much you think your eyes sting, so randomise.
• The onions are food. Cook them for dinner afterwards.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Onions, all the same type	6, similar size	Supermarket	Any variety, but they must all be the same, from the same bag
Sharp kitchen knife	1	Kitchen	Adult use only; a child can use a crinkle cutter on half an onion
Dull knife or a table knife	1	Drawer	For the sharpness comparison, which is a real variable
Chopping board	2	Kitchen	One for the control, one for the wet trial
Swimming goggles	1 pair	Cupboard	Any sealed goggles; glasses do not work
Desk fan or extractor hood	1	Anywhere	A doorway with a breeze through it
Bowl of cold water	1	Kitchen	For the under-water trial
Timer and notebook	1 each	Phone, desk	Essential; this is a measurement, not an impression
Fridge space	for 2 onions	Kitchen	15 minutes in the freezer works instead of 30 in the fridge

Time: prep 35 min including chilling, trials 25 min, analysis 10 min Cost: six onions Mess level: low, and you end up with a lot of chopped onion, so plan a meal around it.

Do the Experiment

Step 1 — Set the conditions and randomise them

Write the six conditions on slips of paper: control at room temperature, chilled onion, cut under water, fan blowing across the board, dull knife, and goggles. Draw them out of a cup in random order. This is the step that makes the data worth anything.

Step 2 — Define the measurement before you start

Agree a scale in advance. Zero means nothing at all, five means you have to stop and walk away. Also record the number of seconds until the first hard blink. Two numbers per trial, written down immediately.

Step 3 — Run the control

Cut a room-temperature onion in half and then into slices, at a steady pace, with the sharp knife. Record both numbers. Then wait three full minutes and rinse the board, because the irritant lingers in the air and will contaminate the next trial.

Step 4 — Run the other five, three minutes apart

Chilled onion straight from the fridge. Onion cut in a bowl of water, carefully, because a wet knife on a wet onion slips. Fan blowing across the board, not at your face. Dull knife. Goggles on. Three minutes between each, same pace, same person.

Step 5 — Repeat the whole set twice more

Three full rounds gives three numbers per condition, which is enough to see whether a difference is real or noise. Average them. Then make dinner with the onions, because you now have a lot of them.

What you should see: The control usually produces a 3 or 4 and a first hard blink within 20 to 40 seconds. Goggles score zero, every time, with no exceptions, and it is slightly infuriating how well they work. The fan is the next most effective, often dropping to a 1. A properly chilled onion typically drops the score by one or two points. The sharp knife beats the dull one by about a point. Cutting under water works well on the eyes and badly on everything else: the onion goes watery and the knife slips. Expect the third round to feel worse than the first regardless of condition, because your eyes are already irritated, which is exactly why randomised order matters.

What’s Actually Going On in the Why Onions Make You Cry Science

An intact onion cell keeps the two halves of this reaction apart. The precursors are amino acid derivatives called S-alk(en)yl-L-cysteine sulfoxides, stored in the cytoplasm, and the enzyme alliinase is held separately in the vacuole. Nothing happens while the cell wall is intact. Eric Block’s 1992 review of the organosulfur chemistry of the genus Allium, in Angewandte Chemie, is the comprehensive account of this family of compounds and remains the standard reference.

Cut the onion and the compartments rupture. Alliinase acts on the main onion precursor and produces 1-propenylsulfenic acid, which is unstable. For most of the twentieth century, textbooks said this unstable intermediate rearranged on its own into the tear-inducing compound.

That turned out to be wrong, and the correction is a genuinely good story to tell a child. In 2002, Imai and colleagues published a paper in Nature titled “An onion enzyme that makes the eyes water”, reporting a second, separate enzyme they named lachrymatory factor synthase. The sulfenic acid does not simply fall apart into the irritant. A dedicated enzyme catalyses that step. So crying at onions is not an accident of unstable chemistry; it takes two enzymes working in sequence, and the plant evolved the second one.

The product is syn-propanethial-S-oxide, listed in PubChem as thiopropanal S-oxide. It is volatile, so it leaves the cut surface as a gas, reaches your eye, dissolves in the tear film and irritates the nerve endings in the cornea. Your tear glands respond by flushing. The compound never touches your eye as a liquid; it arrives through the air, which is why the fan works and why leaning over the board is the worst thing you can do.

That mechanism predicts which remedies should work, and the predictions match the measurements. Chilling reduces the vapour pressure of the irritant and slows the enzymes, so less reaches the air. A sharp knife ruptures fewer cells per cut, so less precursor is released. A fan removes the gas from the path between board and face. Water dissolves the compound before it can evaporate. Goggles simply put a physical barrier in front of the cornea, which is why they work completely.

And the remedies with no mechanism do not work. Holding a spoon in your teeth, keeping bread in your mouth, breathing through your mouth, burning a candle nearby: none of these affect the production of a volatile compound or its path to your eye, and none of them survive a randomised test. That is a useful thing for a child to discover first-hand.

One final piece. Eady and colleagues reported in Plant Physiology in 2008 that silencing lachrymatory factor synthase in onion produced a significant change in the sulfur secondary metabolite profile, which is the scientific basis for breeding low-tear onions. Those exist commercially now. The flavour trade-off is real, because the same sulfur chemistry contributes to onion taste.

How to Explain It to Your Kid

Ages 5-8: The onion builds the sting when you cut it

Tell them the onion keeps two ingredients in separate rooms inside each tiny cell, and cutting knocks the walls down so they mix and make a gas. The gas floats up to your eyes. Then let them put on swimming goggles and watch an adult cut, and ask why the goggles help. “Because it comes through the air” is a complete and correct answer.

Ages 9-12: Enzymes as tools, and a fair test

Introduce enzymes as molecular tools that speed up one specific job. Two of them here, in sequence. Then focus on the experiment design: same onion type, same knife, same person, three minutes apart, random order. Ask them why order matters. Working out that your eyes get more sensitive as you go is a real insight about experimental design.

Ages 13+: Volatility, and why 2002 matters

Go into vapour pressure: a volatile compound’s escape rate rises steeply with temperature, so chilling has a large effect for a small temperature change. Then tell them about the Imai paper. A specific enzyme was found doing a step everyone had assumed was spontaneous, in a vegetable humans have eaten for thousands of years, in 2002. Ask what else might be sitting in a textbook as “it just happens”.

The question to ask: “If I put a cut onion in a sealed jar for ten minutes and then opened the jar right next to my face, would it sting more or less than cutting a fresh one?”

Why Teaching This Early Pays Off

Enzymes are the single most load-bearing idea in biology, and they are usually introduced as an abstraction with a lock-and-key diagram. Here they arrive as the reason your eyes are watering right now. A child who has met alliinase and lachrymatory factor synthase doing a job on a chopping board has something physical attached to the word, which pays off in every biology lesson afterwards, including the enzymes that stop pineapple setting in jelly.

The bigger payoff is about evidence. Five remedies, three of which work and two of which are folklore, in a kitchen, with a randomised order and a repeated measure. That is the structure of a clinical trial in miniature, including the awkward bit: the measurement is self-reported, so expectation contaminates it. Explaining to a child why you shuffle the order is explaining why blinding exists.

And one note on how to handle wrong answers. Smith, diSessa and Roschelle argued in 1994 that student misconceptions are usually partially correct knowledge in transition rather than errors to be deleted. A child who believes the spoon trick works is not being stupid: they have a reasonable theory that something in the mouth matters. Testing it is better than correcting it, and it leaves the belief replaced rather than merely overruled.

Before You Start: What Parents Should Know

Best age	7 to 13. Under 10, an adult cuts and the child runs the timer and the scoring, which is the more interesting job anyway.
Supervision	Constant. The knife is the real hazard here, not the onion.
Real hazards	Sharp knife on a slippery, rolling vegetable. The under-water trial is the most dangerous condition, because a wet knife and a wet onion slip, so an adult should do that one. Onion juice directly in an eye stings sharply but does no lasting harm; rinse with water.
Don’t	Do not test remedies that involve putting anything in or near the eye, and do not use contact lenses during this experiment, because the irritant concentrates in the tear film under a lens.

On eating them: yes, and you should. Six chopped onions is dinner. Everything here is food handled with kitchen equipment, so fry them, roast them or freeze them for later. The only onion worth discarding is the one cut under water, which will have lost flavour compounds into the bowl and gone soggy.

When It Doesn’t Work

• Every condition scored the same. Usually because the trials were too close together. The irritant lingers in the room for minutes, so a three-minute gap and a rinsed board are both necessary. Open a window between rounds.
• The onion barely stung at all, even the control. You have a mild cultivar, or an old onion. Pungency varies enormously between varieties and with how much sulfur the plant had available, and some are bred specifically to be mild. Try a different bag.
• The scores got worse through the session regardless of condition. Expected. Your eyes sensitise. This is exactly why the order must be randomised and why three rounds beat one.
• The chilled onion did nothing. Thirty minutes in a fridge may not have cooled the centre. Cut it in half first, then chill, or use 15 minutes in the freezer. Measure the middle with a thermometer if you have one.
• Cutting under water seemed to make it worse. Likely because you leaned closer to see what you were doing, putting your face directly over the cut surface. The posture change can outweigh the benefit.

Take It Further

Add a distance variable. Cut the same onion at 20 cm, 40 cm and 60 cm from your face, keeping everything else fixed, and score each. The irritant disperses as it travels, so the effect falls off quickly, and this gives a clean result without needing any equipment at all.

Compare varieties properly. Buy four varieties, cut one of each under identical conditions, and score them. Then rate them for taste after cooking. The interesting finding is usually that the mildest one is also the least interesting to eat, which is the real trade-off onion breeders are working with and a far better conversation than a single number. It pairs well with what actually stops apples browning, another enzyme you can interfere with on a chopping board.

What to Watch For Over the Next 3 Months

• Week 4: They put the goggles on without being asked, or move the fan, when an adult starts cutting onions.
• Month 2: The word “enzyme” turns up in a context you did not set up, often about digestion or about fruit going soft.
• Month 3: They question a kitchen remedy they hear somewhere, and ask how you would test it. That transfer, from onions to any folk claim, is the part worth encouraging.

Frequently Asked Questions

Does chilling an onion really stop the crying?

Partly. It reduces how fast the irritant evaporates and slows the enzymes, so in a controlled test it typically drops the sting by one or two points on a five-point scale. It does not eliminate it, and it works better if you halve the onion before chilling.

Why do goggles work so well?

Because the irritant arrives at your eye through the air as a gas. Sealed goggles put a barrier between that gas and your cornea, so nothing reaches the tear film. It is the only remedy that reliably scores zero.

Does holding a spoon in your mouth do anything?

No. There is no mechanism by which anything in your mouth changes the production of a volatile sulfur compound or its path to your eyes, and it does not survive a randomised test. It is the most testable piece of kitchen folklore there is.

Are tearless onions real?

Yes. Research on silencing lachrymatory factor synthase in onion showed it changes the sulfur metabolite profile, and low-tear varieties are now sold commercially. There is a flavour cost, because the same sulfur chemistry contributes to onion taste.

Can my child do the cutting?

From about ten, with a sharp knife, supervision and a stable board. Younger than that, an adult cuts and the child runs the stopwatch and the scoring. Never let a child do the under-water trial, which is the one condition where the knife genuinely slips.

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. Imai, S., Tsuge, N., Tomotake, M., Nagatome, Y., Sawada, H., Nagata, T., & Kumagai, H. (2002). “An onion enzyme that makes the eyes water.” Nature, 419, 685. https://doi.org/10.1038/419685a
2. Eady, C. C., Kamoi, T., Kato, M., Porter, N. G., Davis, S., Shaw, M., Kamoi, A., & Imai, S. (2008). “Silencing onion lachrymatory factor synthase causes a significant change in the sulfur secondary metabolite profile.” Plant Physiology, 147(4), 2096-2106. https://doi.org/10.1104/pp.108.123273
3. Block, E. (1992). “The organosulfur chemistry of the genus Allium: implications for the organic chemistry of sulfur.” Angewandte Chemie International Edition in English, 31(9), 1135-1178. https://doi.org/10.1002/anie.199211351
4. National Center for Biotechnology Information. “Thiopropanal S-oxide, CID 441491.” PubChem Compound Summary. https://pubchem.ncbi.nlm.nih.gov/compound/441491
5. Smith, J. P., diSessa, A. A., & Roschelle, J. (1994). “Misconceptions reconceived: a constructivist analysis of knowledge in transition.” Journal of the Learning Sciences, 3(2), 115-163. https://doi.org/10.1207/s15327809jls0302_1
6. National Research Council. (2012). A Framework for K-12 Science Education. National Academies Press. https://doi.org/10.17226/13165
7. White, R., & Gunstone, R. (1992). Probing Understanding. Falmer Press.