Why Do Onions Make Us Cry?

When we cook, onions are almost always there. But as soon as we start cutting them, our eyes begin to sting and tears roll down. It feels almost personal, like the onion is trying to fight back. The real reason is not magic, but a mix of biology and chemistry inside the onion.

Onions use chemicals as their defense. Since they cannot escape animals or insects, they rely on what is stored inside their cells. When we slice an onion, those cells break open. Enzymes are released and they meet sulfur compounds that are also stored inside. When these two mix, they produce different chemicals, including one that especially bothers us.

That chemical is called syn-Propanethial-S-oxide. It is a type of gas that spreads quickly into the air. When it reaches our eyes, it reacts with the moisture on the surface. Our nerves system senses this irritation and sends signals to the brain. The brain then orders our tear glands to release more liquid, trying to wash the irritant away. That is why we cry, even though nothing sad is happening.

The reaction does not stop at chemistry. Our nerves play a big role, too. The trigeminal nerve, which senses pain on the face, is triggered by the onion gas. Once activated, it causes a burning feeling and forces our eyes to water. This is not something we can control, just like we cannot control sneezing when dust enters our nose.

Not every onion makes us cry in the same way. Sweet onions contain fewer sulfur compounds, making them less painful to cut. Stronger onions have more, so they release more irritants. Recently, scientists even created

onions that do not produce the tear-inducing gas at all. These “tearless onions” are rare, but they show how much science can change even a simple kitchen ingredient.

There are also simple tricks to reduce crying. Cooling onions in the fridge slows down the chemical reaction. Cooking them first can also destroy the enzymes. Some people cut onions under running water or use a fan to blow the gas away. These methods are not perfect, but they make the job easier.

In the end, onions are not trying to make us miserable. They are just protecting themselves with chemicals, and our bodies happen to react strongly to them. Next time we find ourselves tearing up in the kitchen, we can remember that it is not just bad luck. It is a small lesson in how plants defend themselves and how our bodies fight back.