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Tuesday, May 21, 2024

Ask Mr. Scientist: Crying over cut onions and nonstick surfaces

Dear Mr. Scientist,

Why is it that cutting onions makes a person’s eyes water?

—Simon G.

Normally, the different compounds within a cell are kept separate from one another, but once you cut into an onion, you break open its cells and allow the contents to mix together. Among other things, an onion contains an enzyme called allinase and many different sulfurs containing amino acids. Allinase converts these amino acids into sulfenic acid (not to be confused with the much stronger sulfuric acid). This acid is unstable and rearranges itself into a compound called syn-propanethial-S-oxide. This compound is a gas and evaporates into the air—and your eyes. Once in your eyes, syn-propanethial-S-oxide causes the stinging sensation we’re all familiar with, and in response your eyes tear up in an attempt to wash it away.

Dear Mr. Scientist,

If nothing sticks to Teflon, how do they get it to stick to a frying pan?

—Sarah K.

Teflon (whose chemical name is polytetrafluoroethylene or PTFE) is famous for being nonstick and is the second most slippery substance in the world behind diamonds. Getting it to stick to a metal pan requires a few steps. First the bottom of the pan is sandblasted to create tiny crevices that trap the coating and hold it in. To make sure the PTFE really sticks, it’s mixed with a sticky polymer, and a thin layer of this mixture is applied. Although very few things stick to PTFE, it will stick to itself so a second, thicker layer is applied which bonds with the first layer. Once this is done the pan is ready, and any melted messes will slip right off.

Ask Mr. Scientist is written by Michael Leitch. If you have a burning science question you want him to answer, tweet @DC_Science or email it to

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