Why does cutting an onion make us cry

The reason why our eyes weep copiously when peeling an onion is because the onion contains minute pockets of sulphur trioxide, S03(g). Cutting the onion releases this gas. A mammalian eye is covered with a thin film of water-based liquid ( tears ) to minimize friction with the eyelid. The tears occur in response to SO3 dissolving in this layer of water to form sulphuric acid  

The sulphuric acid produced dissociates in the water to form SO42- and two protons. The eyes sting as a direct consequence of contact with this acid. 

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