十万个为什么·科学启蒙30篇（2）
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Why Do Onions Make You Cry When You Cut Them?
为什么切洋葱时会流泪？
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When you cut an onion, cells break open and release enzymes that mix with sulfur compounds.
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These reactions create a volatile gas called syn-propanethial-S-oxide.
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The gas rises and dissolves in the moisture of your eyes to form mild sulfuric acid.
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This irritates nerve endings, triggering tear glands to flush out the chemical.
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Chilling onions before cutting slows down the enzyme activity and reduces gas release.
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Using a sharp knife causes less cell damage and therefore fewer tears than a dull one.
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Wearing goggles or cutting near a running fan helps carry the gas away from your face.
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Some onion varieties naturally contain less sulfur, making them 'tear-free' by design.
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Scientists have even bred low-lachrymatory onions using gene-editing techniques.
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Though annoying, this defense mechanism evolved to deter insects and animals from eating the plant.