双语：为什么我们在切洋葱时会流泪？

　　Researchers have uncovered how a compound in onions makes our eyes water when they’re cut.

　　研究人员们揭露了洋葱内部某种化合物在我们切洋葱时是如何使我们流眼泪的。

　　When an onion is cut, it has a natural defense mechanism that springs into action, producing a compound that irritate the eyes, but researchers weren’t sure about how this mechanism worked - until now.

　　当一个洋葱被切时，它的天然防御机制会立马产生作用，产生一种刺激双眼的化合物，但是直到现在，研究人员们仍然不确定这一机制是如何起运作的。

　　By analyzing the crystal structure of a chemical in the onion, a new study has found exactly how a particular enzyme in onions helps the tear-inducing compound take effect.

　　通过分析洋葱内某种化学物质的晶体结构，一项新的研究准确发现了洋葱内某种特定的酶是如何帮助催泪化合物起作用的。

　　According to the National Onion Association, the average American consumes 20 pounds (9 kilograms) of onions each year.

　　据国家洋葱协会介绍，美国人平均每人一年吃掉20磅(9公斤)洋葱。

　　When an onion is cut, it releases a compound called lachrymatory factor (LF), which makes the eyes sting and water.

　　当一个洋葱被切的时候，它会释放一种叫催泪因子的化合物，这会刺激眼睛流泪。

　　LF is a rare compound, and only four known natural types exist.

　　催泪因子是一种罕见的化合物，目前只知道存在四种天然类型。