Why do onions make you cry?
Cutting an onion breaks its cells and releases a gas that irritates your eyes. Your eyes respond by making tears to wash it away.
Going deeper
Onions store sulfur compounds and enzymes in separate parts of their cells. When a knife breaks the cells open, they mix and produce a volatile gas called syn-propanethial-S-oxide, which drifts up into your eyes.
A sharp knife crushes fewer cells and releases less gas. Chilling the onion first slows the reaction, and cutting near a fan or open window helps carry the gas away.