Why You Cry When You Cut Onions

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While cutting an onion, substances containing sulfur gas is released into the air. When it gets into the eyes, it reacts with water which naturally lubricates the eye, and forms small amounts of sulfuric acid which is extremely irritating.

As a defense mechanism, we cry! The body's response is to send stimuli to the tear glands to produce tears in an attempt to dilute and wash away the acid that is in contact with the surface of the eye (the cornea).