Why do onions make us cry?

Onions absorb sulfur from the soil and store it as compounds inside their cells. When you slice an onion, you rupture those cells, releasing enzymes that convert the sulfur compounds into a volatile gas called syn-propanethial-S-oxide. This gas drifts up to your eyes, dissolves in the thin layer of tear film, and forms a small amount of sulfuric acid. Your eyes sting, and the lachrymal glands flood them with tears to wash the irritant away. Chilling the onion slows the reaction, and a sharp knife ruptures fewer cells, so both tricks reduce the tears.