Onions are a delicious and versatile vegetable that is used in many different dishes. However, onions also have a reputation for making people cry. Onions contain a chemical called syn-propanethial-S-oxide, which is released when the onion is cut. This chemical irritates the eyes, causing them to water. The more an onion is cut, the more syn-propanethial-S-oxide is released, and the more likely it is that someone will cry. A Cute Crying Onion Sticker.