Question

why we get tears while cutting onions?

Answer 1

We get tears while cutting onions because onions produce the chemical irritant known as syn-propanethial-S-oxide. It stimulates the eyes' lachrymal glands so they release tears. 

Answer 2

Lachrymatory-factor synthase is released into the air when we cut an onion. The synthase enzyme converts the amino acids sulfoxides of the onion into sulfenic acid. The unstable sulfenic acid rearranges itself into syn-ropanethial-S-oxide. Syn-propanethial-S-oxide gets into the air and comes in contact with our eyes.