Why Onions Make Us Cry

Anyone who has ever diced up onions knows the familiar sting of tears that come with the territory. This reaction is caused when the sulfuric acid, sulfur dioxide, and hydrogen sulfide are released from the onion upon slicing it open. This combination makes up the unstable compound known as syn-propanethial-s-oxide. The compound turns into a gas, which interacts with the moisture in your eyes to cause a burning sensation. When the brain senses an irritant in the eye, it sends out a message to flush it out – hence, the tears. There are a few methods for preventing this reaction. Some people say chewing gum helps, but the most effective trick is actually to burn a candle next to the onion while you’re cutting it. The flame will “suck up” the gaseous chemicals put out there by the onion.