十万个为什么·科学启蒙30篇（2）

17 / 30

为什么切洋葱时会流泪？

When you cut an onion, cells break open and release enzymes that mix with sulfur compounds.
These reactions create a volatile gas called syn-propanethial-S-oxide.
The gas rises and dissolves in the moisture of your eyes to form mild sulfuric acid.
This irritates nerve endings, triggering tear glands to flush out the chemical.
Chilling onions before cutting slows down the enzyme activity and reduces gas release.
Using a sharp knife causes less cell damage and therefore fewer tears than a dull one.
Wearing goggles or cutting near a running fan helps carry the gas away from your face.
Some onion varieties naturally contain less sulfur, making them 'tear-free' by design.
Scientists have even bred low-lachrymatory onions using gene-editing techniques.
Though annoying, this defense mechanism evolved to deter insects and animals from eating the plant.