It is not the strong odour of the onion that makes us cry, but the gas that the onion releases. The onion contains oil and sulfur which irritates both our nose and eyes. Cutting an onion arouses a gas called propanethiol Soxide, which then couples with the enzymes in the onion to emit a passive sulfur compound. When this upwardly mobile gas encounters the water produced by the tear ducts in our eyelids, it produces sulfuric acid. In response to the caustic acid, our eyes automatically blink and produce tears. The only way to get rid of this phenomenon is to boil the onion before cutting it.