How Onions ACTUALLY Make You Cry (And How to Stop Them)

TL;DR

Onions trigger crying via a chemical chain reaction, and a 2025 Cornell study reveals that a sharp, slow-cutting knife minimizes the irritant droplets reaching your eyes.

Key Points

• 1.Onions produce syn-propanethial-S-oxide through a two-step enzyme reaction. Cutting breaks cell walls, releasing alliinase to create 1-propenesulfenic acid, which then reacts with a second enzyme to form the tear-inducing compound — a process only fully understood in 2002.
• 2.A 2025 Cornell University study examined onion-cutting physics using a precision guillotine, high-speed cameras, and an electron microscope. They tested how knife sharpness, cutting angle, speed, and onion temperature affected the volume and trajectory of irritant droplets released.
• 3.Blunt knives and faster cutting produce significantly more irritant droplets. A dull blade compresses the onion membrane longer before breaking it, causing a larger burst; cutting quickly also releases more droplets, making a slow, sharp knife the best defense.
• 4.Chilling onions before cutting actually worsens droplet release, debunking a common hack. Cold tissue becomes stiffer, absorbing more pressure before bursting with greater energy; droplets also fly upward, meaning a nearby water bowl can only catch bounces, not the initial spray.