Question

why onion cause tear from eye when we cut it?

Answer 1

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. The lachrymal glands become irritated and produces the tears!

Answer 2

Hey mate onion produces a gas known as syn-propanithial -s-oxide which which stimulates the lachrymal glands so they release tears