Cutting an onion often leads to watery eyes within seconds, even though the vegetable itself does not physically injure the eyes. The reaction is caused by a chemical released when onion cells are damaged during cutting. The resulting irritation activates the eyes' natural protective response, producing tears that help flush the irritant away.

Onions contain different compounds that are separated within their intact cells. When an onion is cut, its cells are broken and an enzyme called alliinase comes into contact with other compounds. This triggers a series of chemical reactions that ultimately produce a volatile substance known as syn-propanethial-S-oxide, commonly referred to as the lachrymatory factor.

Because this compound is volatile, it quickly travels through the air and reaches the surface of the eyes. When it comes into contact with the moisture covering the eyes, it causes irritation to the sensory nerves in the cornea. The brain responds by activating the tear glands, resulting in the familiar watering that occurs while chopping onions.

Also Read: Why Do We Close Our Eyes While Sneezing? The Science Behind This Reflex

The tears are essentially part of the body's defence system. Their job is to dilute and remove irritating substances from the surface of the eyes. This is why cutting onions can cause a strong reaction even though the effect is generally temporary and does not normally cause lasting harm to healthy eyes.

The amount of irritation can vary depending on the onion and how it is cut. A sharper knife can cause less crushing of the onion's cells, potentially reducing the amount of irritating compounds released. Chilling an onion before cutting can also slow the movement and release of some volatile compounds, although it does not completely prevent the reaction.

So, the familiar riddle about an onion "making your eyes water" has a straightforward scientific explanation. Cutting damages the onion's cells, triggering a chemical reaction that produces an airborne irritant, while the eyes respond by producing tears to protect and clean themselves.

Also Read: Snickers Was Named After A Horse: The Surprising Story Behind The Chocolate Bar