Onions make you cry because when they are cut, they release a volatile gas called syn-propanethial-S-oxide. This gas irritates the eyes by reacting with the water layer on the eyes to form sulfuric acid, which triggers the nerves to send pain signals to the brain. In response, the brain stimulates tear glands to produce tears to flush out the irritant. This reaction is actually a defense mechanism of the onion to deter animals from eating it. Here's how the process unfolds:
- Cutting an onion breaks its cells and releases enzymes and sulfur-containing compounds.
- These compounds mix and form syn-propanethial-S-oxide gas.
- The gas reaches the eyes and reacts with the moisture there, causing irritation.
- The eyes respond by producing tears to wash the irritant away.
Different types of onions have varying amounts of sulfur compounds, so some make you cry more than others, with yellow onions generally causing the most tears. There are also tearless varieties of onions that have been bred to reduce this effect. This defensive chemical reaction is why the simple act of chopping onions often results in teary eyes.
