Biology
What causes that strong smell when you cut an onion?
Quick fact
Onions release a gas called syn-propanethial-S-oxide (aka 'lachrymatory factor') that dissolves in your tears to form sulfuric acid, triggering a burning sensation that makes you cry.
Why this is interesting
An onion's tear-inducing compound is a volatile chemical weapon designed to deter pests—but it's not produced until you break the cell walls, creating a new compound that attacks your eyes on contact.
Read the full explanation
Understanding What causes that strong smell when you cut an onion?
When you cut an onion, you rupture cells, allowing an enzyme (alliinase) to mix with sulfenic acids, which rapidly rearrange into the volatile lachrymatory factor. This gas wafts upward and reacts with the moisture in your eyes.
A deeper explanation
This chemistry was an evolutionary invention: the compound repels insects and animals from eating the bulb. Scientists have even identified the exact gene responsible (the 'lachrymatory factor synthase' gene), and by silencing it, they've created tearless onions. The tears you shed aren't sadness—they're your eyes literally trying to dilute and wash away a mild acid. Interestingly, chilling onions before cutting slows the enzyme reaction, and cutting near a flame or running water captures the gas before it reaches your eyes. So every tear is a testament to millions of years of plant-animal warfare, where your kitchen counter is the battleground.