Why Do Onions Make You Tear Up?

My friend J asked me why onions make her cry. Well, I know it has something to do with a chemical reaction when you cut it up but I don't know how to explain it to her in details. So I made my own research and here's what I found:

When you cut into an onion, you rupture its cells, releasing enzymes that produce a gas called propanethial sulfoxide. Once that gas reaches your eyes, it reacts with tears to produce a mild sulfuric acid. And that hurts. The brain then signals the eyes' tear glands to produce more liquid to flush the stuff out. The more you chop, the more irritating gas you produce and the more tears you shed. "The onion's chemical reaction is a defense mechanism that evolved to repel pests," explains University of Wisconsin—Madison horticultural professor Irwin Goldman, PhD.

Keep the stinging and crying to a minimum by chilling an onion in the freezer before cutting it; cold temperatures slow release of the enzymes. The highest concentration of enzymes is at the bottom of the onion, so cut it last to postpone the weeping (and the irritation) for as long as possible.