Question

why do we cry when we cut an onion

Answer 1

Onions produce the chemical irritant known as syn-propanethial-S-oxide. It stimulates the eyes' lachrymal glands so they release tears. Scientists used to blame the enzyme allinase for the instability of substances in a cut onion. ... The synthase enzyme converts the amino acids sulfoxides of the onion into sulfenic acid.

Answer 2

we don't cry our tears cones out from our eyes to heel or to protect the eyes from the chemical released during cutting of onion