Why do onions make you cry?

You ask: Why onion make cry?

Onion keep two things apart inside its cells, like keeping fire-flint and dry grass in separate pouch. One is a smelly sulfur stuff, other a little worker (enzyme). When Caveman knife cut the onion, cells burst and the two mix — a quick reaction puff out a stinging gas into the air. Gas float up, reach Caveman eye, touch the wet film there and turn into a weak acid. Eye feel the sting, ring the alarm, and flood with tear to wash the bad stuff away. That why Caveman cry, even though not sad at all. Cut the onion under cool river water, or chill it in the cold about half an hour first, and less gas reach eye. Cave wisdom: onion not make Caveman sad — it fire a tiny sting-gas at the eye, and the tear just Caveman eye rinsing it out.