The vegetable known for its ability to make you cry is the onion. When an onion is cut, it releases a gas called syn-propanethial-S-oxide. This compound irritates the eyes, leading to the production of tears as a protective response. The reaction occurs when the gas comes into contact with the moisture in the eyes, forming sulfuric acid, which causes a burning sensation. Various methods, such as chilling the onion before cutting or using a sharp knife, can help minimize this tear-inducing effect, but the chemical reaction is an inherent characteristic of onions.