There are no dumb questions on this blog, my friend! I’ll bet a bunch of people are currently reading this thinking MAN I’D REALLY LIKE TO KNOW THAT TOO.
So, let’s talk CHEMISTRY. When onions are grown, they absorb sulfur from the earth, which creates a kind of volatile, organic molecule called amino acid sulfoxides. These form sulfenic acids in the onion cells, and they’re kept separate from enzymes (complex proteins that cause chemical changes). By cutting an onion, you’re actually breaking its cells—so enzymes are now free to mix with the acids. Together they form a sulfur gas called propanethiol S-oxide, and this wafts right up towards your eyes without mercy.
When it reaches them, the gas reacts with the water in your eyes and forms a mild sulfuric acid, which is what causes the REALLY irritating pain. As a defence mechanism, you reflexively tear up to wash the irritant away.
So when you cry, it’s not due to being emotional about cooking, it’s because you have SULFURIC ACID IN YOUR EYES.
That’s pretty badass.