Book 1, Question 3
Why Do Onions Make You Cry? The Tiny Gas Trick!
Can a vegetable really make you cry? When an onion is cut, it releases a sharp gas called syn-propanethial S-oxide, the onion's way of defending itself. The gas drifts up, mixes with the moisture in your eyes, and your tears rush in to wash it away. Chilling an onion in the fridge for about 30 minutes before cutting slows the gas down, so it stings less. Ask a grown-up to do the cutting!
From the book
Page from A Million Whys Book 1
More fun facts
- People have been eating onions for at least 5,000 years, and they have been making cooks cry the whole time.
- The tears are a kind of plant defense. Scientists describe it as chemical warfare that helps keep hungry animals away.
- Cutting an onion breaks its cells. That lets an enzyme called alliinase mix with sulfur-based molecules and start a chain reaction.
- In the early 2000s, scientists at a Japanese food company found a second enzyme in onions. It is called LF synthase, and it makes the eye-stinging gas.
- To see what that enzyme looks like, researchers grew it into crystals, froze them in liquid nitrogen, and mapped them with X-rays.
- Onions are not the only plants with alliinase. Garlic makes it too.
- Scientists have found ways to grow less tearful onions in the lab, but those ideas have not reached grocery stores yet.
Sources: Smithsonian Magazine, Science News Explores, Cleveland Clinic
Questions kids ask
Why does chilling an onion help?
Cold slows down the release of the stinging gas, so less of it reaches your eyes while you cut.
Which enzyme makes the tear gas?
An enzyme called LF synthase. Scientists at a Japanese food company discovered it in the early 2000s.