Unless you've managed to avoid cooking entirely, you've probably experienced the burning and tearing from vapors that cutting up an onion produces.

Cutting an onion bursts its cells, producing a chemical process which releases the contents of those cells into the immediate atmosphere, causing you to tear up as you slice and dice.

Acid Effect

Amino acid sulfoxides form sulfenic acids as you slice into an onion. These enzymes which were isolated are now free to mix with the sulfenic acids to produce ​propanethial S-oxide, a volatile sulfur compound gas which wafts upward and into your eyes. This gas reacts with the water in your tears to form sulfuric acid. The sulfuric acid burns, stimulating your eyes to release more tears to wash the irritant away.

Stop Crying

There are a few ways to curb the chemical process which causes you to cry when cutting an onion, including:

Cook the onion. Cooking inactivates the enzyme, so while the smell of cooked onion may be strong, it doesn't burn your eyes.

Wear safety goggles or run a fan. This actually prevents the vapors from the compound entering your eyes or blows the compound vapors safely away.

Refrigerate the onion before cutting. Cooling slows reactions and changes the chemistry inside the onion. The same effect can be accomplished by cutting the onion under water.

Use stainless steel. The sulfur-containing compounds also leave a characteristic odor on your fingers. You may be able to remove or reduce some of the smell—and tears—by wiping your fingers on a stainless steel odor absorber.

Other Methods

A few more proven methods for avoiding the waterworks while cutting into or preparing an onion involve cooking prep methods such as locating the root, removing the bulb and even slicing lengthwise before dicing.

So, take heart: With a little preparation and an understanding of basic chemistry, you can slice, dice and cook an onion without ever shedding a tear.