The Scientific Answer
When you cut an onion, you break open its cells and release a mix of chemical compounds. One of the main culprits is an enzyme called alliinase. This enzyme reacts with the onion’s sulfur compounds, producing a gas known as syn-propanethial-S-oxide. Because it is volatile, the gas spreads easily through the air.
When the gas reaches your eyes, it mixes with the moisture on their surface and forms a mild acid: sulfuric acid. This irritates the nerve endings in your eyes, creating that familiar stinging sensation. In response, your tear glands produce tears to flush out the irritant and protect your eyes.
In short: onions make you cry because cutting them releases a gas that rises toward your eyes. When it mixes with your tears, it creates an irritating substance, so your eyes produce even more tears to protect and clean themselves.
How to Cut Onions Without Crying
- Soak the onion in water for five minutes before cutting it.
- Chill the onion: lower temperatures slow down the chemical reactions.
- Cut it under running water: water dissolves the gas before it reaches your eyes.
- Use a sharp knife: a clean cut damages fewer cells than a rough one.
- Wear glasses or contact lenses: they act as a physical barrier.
Did You Know?
The heaviest onion ever grown weighed 8.97 kg (19 lb 12.4 oz) and was grown by Gareth Griffin from the United Kingdom.
