Why do tears uncontrollably stream down when cutting onions?

It's the onion's 'chemical weapon' fighting back against your cornea nerves! When you slice an onion with a knife, you violently rupture the onion cells' cell walls. This causes the 'alliinase' and sulfur-containing amino acids, which were previously separated, to mix instantly. Within milliseconds, they release a volatile gas called 'syn-propanethial S-oxide'. Once this gas diffuses and comes into contact with the tear film on your eye's surface, it instantly converts into trace amounts of 'sulfuric acid'! The corneal nerves on the surface of your eyeball sense this acidic irritation and immediately send a top-level 'chemical crisis alert' to your brain. The brain then commands the 'lacrimal glands' around your eyes to produce a massive amount of tears, attempting to 'dilute and wash away' these acidic substances. That's why you end up crying rivers of tears while chopping onions!