Chopping onions makes you cry because when you cut an onion, you break its cells and release sulfur-containing compounds. These compounds, through a series of chemical reactions, form a volatile gas called syn-propanethial-S- oxide. This gas evaporates and comes into contact with the moisture in your eyes, forming sulfuric acid, which irritates the eyes. In response, your brain triggers tear production to wash away the irritant, causing your eyes to water and sting.
The essential process is that the enzymes inside the onion interact with sulfur-rich amino acids to produce unstable sulfenic acids, which rearrange into syn-propanethial-S-oxide. This chemical defense mechanism is how onions protect themselves from predators.
To reduce crying, people use methods such as chilling onions before cutting, using a sharp knife to minimize cell damage, cutting under water, or wearing goggles to block the gas from reaching the eyes.
