Revealed: The reason onions make us cry
The reason we tear up when cutting onions is because of a chemical reaction similar to that caused by tear gas.
The Library of Congress explains the process. When we cut onions, lachrymatory factor synthase is released. The synthase enzyme turns the onion's amino acid sulfoxides into sulfenic acid. The sulfenic acids spontaneously rearrange to form the chemical that triggers tears, syn-propanethial-S-oxide. When this gets into the air, we start crying.
Some say putting an onion the fridge will help slow down the reaction, and fewer chemicals are released when using a sharp knife because there will be less cell wall damage. If tearing up is that big of an issue, just give onions the chop from your next recipe.