Question

whilecutting onions our tears start flowing

Answer 1

It turns out when you cut an onion, it releases a gas called, ready for this, Propanethiol S-oxide. When mixed with certain enzymes in the onion, it creates a sulfur gas. These gases then get to your eyes and create a mild acid which irritates the eyes.
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Answer 2

because the onions contain sulfuric acid
to protect the eyes from the chemical our eyes produce tears which wah away them