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🧅 Why onions make you cry, and how to stop it

A cut onion builds a tear-gas molecule in seconds. Chemists finally caught the enzyme responsible in 2002, and it explains every kitchen trick.

lessons

~10 min

to learn

🔬 Science

subject

Adults

level

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What you’ll learn

A weapon with a two-part fuseExplain how cutting an onion mixes a stored sulfur compound with enzymes to create a volatile irritant.Two compartments, two enzymes and one cut: alliinase makes an unstable intermediate and a second enzyme turns it specifically into the tear compound, which garlic lacks.
Why your eye, and not your handDescribe how a volatile irritant triggers the corneal nerves and the tear reflex, and why pungency varies between onions.The compound travels as a gas to the one wet, exposed, nerve-rich surface you have, and the strength of the effect depends on variety and on sulfur in the soil.
What actually worksEvaluate common remedies against the mechanism and explain why cooking ends the problem entirely.Sharp knives, cold onions, running water and moving air all intervene at a real step, cooking destroys the enzyme outright, and remedies with no mechanism do nothing.

Questions this course answers

Why does an intact onion not make you cry?

The tear compound does not exist in a whole onion. Cutting mixes an enzyme with a stored sulfur compound, and the irritant is manufactured in seconds by that reaction.

Put the chain of events in order, from knife to tears

Two enzymes act in sequence, and only the second one produces the irritant. Without it the intermediate becomes ordinary flavour molecules instead.

Why does a sharp knife reduce the sting?

Everything starts with broken cells. A blunt blade crushes a wide band of tissue on either side of the cut and releases far more of the reactants.

Match each remedy to the step in the mechanism it interferes with

Useful remedies all intervene somewhere real: fewer broken cells, slower chemistry, less vapour reaching you, or no working enzyme at all.

In your own words, why do your eyes sting while your hands, which are actually touching the onion, feel nothing?

It is an airborne attack on the one exposed, wet, nerve-rich surface you cannot close in time.

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