When you cut an onion, you break cells that release enzymes. These enzymes produce a gas that turns into sulfuric acid when it hits the moisture in your eyes.
The Chill Factor
Place your onions in the freezer for 15 minutes before cutting. The cold slows down the enzymatic reaction, reducing the amount of gas released.
Sharpness Matters
A dull knife crushes cells, while a sharp knife slices through them cleanly. Fewer crushed cells means less gas and fewer tears.