Why do onions make you cry?

Because they cannot crack a joke.

No.

Onions consist of amino acid sulfoxides.

When we cut an onion, millions of onions cells rupture.

Releasing the amino acid sulfoxides along with some special enzymes.

These special enzymes react with amino acid sulfoxides to form a chemical called Syn-propanethial-S-oxide.

This chemical is volatile, that is, it easily evaporates at normal temperature, forming a gas.

When this gas reaches our eyes.

It reacts with the substance that keeps our eyes lubricated and forms mild sulfuric acid.

What?

An acid in my eyes.

Absolutely.

This sulfuric acid gives us a burning sensation.

Now, in order to wash off this acid, our lacrimal glands produce a disinfecting liquid.

But when our eyes cannot hold any extra amount of disinfecting liquid.

It starts to fall down, making us cry.