Why do onions sting your eyes and make you cry when you dice them?  Is there anything that can be done to prevent this?

Whenever you cut an onion, you slice through the cells, allowing certain sulfur-based chemicals to come into direct contact with an enzyme which breaks them into smaller molecules.  Some of those molecules are volatile, meaning that they are easily dispersed into the air where they can be carried into your eyes.  These chemicals react in two ways in your eye.  First, they directly stimulate nerve endings in the tear glands.  Second, they dissolve in your tears to create mild sulfuric acid, which is also irritating.

There are any number of old wives' remedies on how to prevent this uncomfortable experience, including holding a matchstick in your mouth, holding a burnt matchstick in your mouth, cutting the onion underwater, chewing on bread or raw onions, leaving the skin on, and not cutting the root end.  While cutting the onion underwater may help, it does make for a soggy mess when you try to use it afterward.  The other suggestions are more psychological than scientific.

There are, however, a few suggestions that might really help:

• Use a sharp knife, which will minimize the amount of cell damage.  I have never seen an actual comparison on this, but it stands to reason that a sharp knife will damage less surrounding tissue than a dull knife.  The difference may be small, however.
• Refrigerate the onion beforehand.  This slows down the action of the enzyme, which in turn reduces the amount of the volatile chemicals that are released.  Alternately, you could soak the onion in ice water for a half hour or so.
• Wear goggles.  If the burning from the onions is a real problem, keep a pair of swim goggles close by and put them on while you chop the onions.
• Set up a fan so that it blows most of the volatile sulfur compounds away from your face so they can't get into your eyes.

For those who care, the lachrymator is thiopropanal sulfoxide which is one of the chemicals produced when the enzyme alliinase reacts with various sulfur compounds, found in onions, and which are similar to the amino acid cysteine.