India, Sept. 3 -- "If you cut me, I won't cry, but I will make your eyes water. What am I?" Almost everyone knows the answer to this riddle: an onion. But why is it that cutting an onion makes our eyes well up with tears? Here is the explanation.

Onions are bulbs that mature underground. They are particularly good at absorbing sulfur, which they use to synthesise several amino acids.

When we cut an onion, we break open numerous cells whose contents come spilling out. It results in the previously separated enzymes meeting the sulfur-rich amino acids and forming unstable sulfenic acids, which then rearrange into a chemical known as syn-propanethial-S-oxide.

The final chemical is a volatile compound, which vaporises, floats up from the ch...