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.