Why do onions make you cry?

Onions make you cry because of the cell membranes. When cut, they act almost like a defense mechanism. The easiest way to get around it is to use chilled onions and a really sharp knife to damage the membranes less :)

When you slice through an onion, you break open a number of onion cells. Some of these cells have enzymes inside of them, and when they are sliced open, the enzymes escape. The enzymes then decompose some of the other substances that have escaped from sliced cells. Some of these substances, amino acid sulfoxides, form sulfenic acids, which then quickly rearrange themselves into a volatile gas. The gas reaches your eyes and reacts with the water that keeps them moist. This changes the chemical's form again, producing, among other things, a mild sulfuric acid, which irritates the eyes. The nerve endings in your eyes are very sensitive and so they pick up on this irritation (this is why our eyes sting when we slice onions). The brain reacts by telling your tear ducts to produce more water, to dilute the irritating acid so the eyes are protected. Your other reaction is probably to rub your eyes, but this will actually make the irritation a lot worse, of course, if you have onion juices all over your hands. This is also what gives the onions their great taste!

Because they break my heart - EVERY single time!

As you chop them they release a mild form of hydrochloric acid which irritates your eyes.

If you think it's bad now, just wait until the gases are recycled through the intenstinal tract and just hope you are not in a closed in area with the person who ate them. Really make your eyes water. LOL! ROFL!

They contain lots of acid

Well, there are already some great answers to why onions make us cry, but if you don't want to cry...? Light a candle and place it on the kitchen counter, close to where you are cutting onions. This stops the onion 'vapors' from getting to your eyes. It works really well!