Onions typically grow in the ground, which means they absorb sulfur. When an onion is sliced, it releases a sulfur enzyme into the air known as syn-Propanethial-S-oxide. When that enzyme mixes with the natural moisture of the eye, it becomes an irritant.

"In response to the irritant, the eye sends a signal to the brain, which in turn sends a signal to the lachrymal gland, which rests just above the eye," explains Scott Barnes, O.D., optometrist at Alta View Eye Care. "The gland then washes water down into the eyes to flush out the acid… which explains the tears."

How can you prevent the tears?

There are a few ways to reduce the amount of enzymes that get released into the air when cutting an onion:

• Cut the onion underwater
• Chill the onion before cutting
• Wear goggles

According to Dr Barnes, one of the best methods is to use a sharp knife and avoid the core of the onion, since more of the eye-irritating enzymes are stored in the center. Cut the sides first, then finish at the core. 

KUTV's Matt Gephardt answered this question as part of a Good Question segment.