Everyday science

Why do onions make you cry?

An onion sitting on the counter is harmless. Cut it, and within seconds your eyes sting and water. That's not bad luck — it's a defence system, and your knife is the trigger.

The onion is booby-trapped

A whole onion keeps two things in separate compartments inside its cells: certain sulphur compounds, and enzymes. Apart, they do nothing. Your knife smashes the cells open and lets them mix — and that's the spark for a tiny chemical chain reaction.

This is the onion's defence: in nature, it discourages animals from chewing on it. You just happen to be doing the chewing with a blade.

The tear gas it makes

That reaction produces a small, volatile molecule with a mouthful of a name — *syn-propanethial-S-oxide*. It floats up off the cut onion as a gas and drifts toward your face.

When it reaches the wet surface of your eyes, it reacts with the moisture and forms a mild sulphuric acid. It's a tiny amount, but your eyes don't like it one bit — so they sound the alarm.

Why the tears come

Your eyes have nerve endings that detect irritants. The moment they sense the acid, they trigger your tear glands to flush the irritant out with a flood of water. The crying isn't emotional — it's a windscreen-wiper reflex, washing the chemical away.

How to actually stop the tears

Since the culprit is a gas that loves to reach your eyes, the tricks all aim to slow it down or move it away:

• Chill the onion for 15 minutes first. Cold slows the reaction, so less gas escapes.
• Use a sharp knife. A clean cut ruptures fewer cells than a dull, crushing one — less mixing, less gas.
• Cut near running water or a fan, which carries the gas away from your face.
• Leave the root end for last — it holds the highest concentration of the sulphur compounds.

So the onion isn't trying to make you sad. It's defending itself with a puff of homemade tear gas — and your tears are simply your eyes doing their job.