I also shared it on my personal Facebook page. This is from the book Fifty Plants that Changed the Course of History by Bill Laws. (I’m only up to page 26 but so far it’s fascinating.) From page 10:
Cut into an onion, and a chemical — thiopropanal-S-oxide — is released. It is a substance that reacts on the eye like pepper spray: we are reduced to tears… American biochemist William Frey discovered that while all tears are composed of water, mucus, and salt, tears of sadness contain additional proteins, suggesting that when people cry the body is having an emotional clear-out of stress-related chemicals. Shedding real tears is good for you.