Onion.

I love cooking with onions. They add so much flavor to everything and I always have a couple on hand because I'm using them every time I cook. But this morning as I was making my marinade and chopping up the onion, something happened that has never happened before! My eyes started burning and watering! Why does that happen??

sulfuric acid!

When you cut an onion, you are slicing cells that release enzymes that mix with the air and produce propanethiol S-oxide which is a sulur compound that goes up into your eyes. This gas reacts with the water in our tears to form sulfuric acid... which burns... and causes your eyes to tear to wash away the irritant.

So now you know.