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Onions make us cry because they have pungent juice that sprays up into your eyes. Lachrymatory factor irritates the eyes of a person chopping, it stimulates the tear glands to produce tears, like tear gas.

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There are small particles at the onion and when it mixed in the air and reach your eye, the small particles will make you cry because of the irritaion of the eye, sorry I cant remember the name of that particle, but hope this helps