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The onion itself contains oil, which containssulfur, anirritantto both our noses and to our eyes. Cutting an onion arouses a gas contained within the onion, propanethiol S-oxide, which then couples with theenzymesin the onion to emit a passive sulfur compound. When this upwardly mobile gas encounters the water produced by thetear-ductsin our eyelids, it produces sulfuric acid.

In response to the caustic acid, our eyes automatically blink, and produce tears which irrigate the eye, and which flush out the sulfuric acid.

Another reflex to rid the eyes of a foreign substance, that of rubbing our eyes with our hands, oftenexacerbatesthe situation, because our hands are coated with the caustic, sulfuric acid producing oil from cutting the onion, which we then rub directly into our eyes.

Much to our chagrin, the only remedy for ridding the onion of its pungent, irritating oil is to boil it, not to slice it ordiceit.