Before we get to the answer, it's important to know why we tear up when cutting raw onions in the first place.When onions grow underground, they absorb the sulphur from the earth, which creates a class of volatile organic molecules called amino acid sulfoxides. Crushing the onion cells under the pressure of a knife releases the amino acid sulfoxides to form sulfenic acids, which then reacts with the air to create a volatile sulfur compound that wafts towards your eyes. When the gases hit your eyes, they create a burning and stinging irritant in your eyes, and your eyes naturally tear up in an effort to wash the irritating sensation away.The most effective way to cut an onion without shedding tears (other than to buy them pre-chopped) is to wear swim goggles. If the idea of looking like an Olympic swimmer in the kitchen is too silly for you, you can try chilling the onion before cutting, lighting a candle nearby, sticking a piece of bread in your mouth, or running a fan that will blow the onion fumes away from your face.