Science Lesson: Here's Why Onions Make Us Cry

Onions, wrote the poet Pablo Neruda, "make us cry without hurting us."

Neruda is moved by the vegetable's bulbous beauty, and Shrek relates to the onion's possession of layers on an emotional level.

But scientists have figured out why onions have the less contemplative of us tearing up when we cut them open.

It's because of chemical syn-propanethial-S-oxide, according to chemist and onion expert Eric Block.

"See, the onion is a perennial bulb that lives in the ground with lots of critters who are looking for a snack," Block said to NPR. "So it has evolved a chemical defense system."

The onion - that clever little Allium - contains vacuoles in each of its cells that release chemicals when cut or bitten open. 

When they're released, Block said, "a whole cascade of chemical processes happen within an instant."

That chemical cascade causes tiny syn-propanethial-S-oxide molecules to float into the air and pierce the eyes of the consumer like stinky daggers. 

Our tears, then, are no accident by the onion, which attempts to deter predators with its eyeball-burning properties. 

Even still, it's the most widely cultivated species of its genus and boasts a variety of health benefits. Clearly, crying won't stop people from chopping up the veggie for some extra flavor and crunch. 

Fortunately, Block has a few suggestions to keep eyes nice and dry. 

Most effective is to chop onions near a fan, so air blows the molecules away from the face.

Another technique is to refrigerate the vegetable a few minutes before chopping. That way, the molecules will be cold and move to your face a bit less aggressively. 

Some cooks swear by wearing goggles or chewing gum while slicing onions, but Block said that looks silly and doesn't keep chemicals from getting to the mouth or nose. 

Despite these methods, Block said it's almost impossible to keep the eyes from watering when using onions.

"Look at you, chopping and weeping. Idiot," wrote Suji Kim in Monologue for an Onion.

Perhaps there's something to be appreciated in the onion's unique design and odor.