When you chop an onion, you are actually setting off a sophisticated chemical defense mechanism. Onions contain amino acid sulfoxides that form sulfenic acids when cells are damaged. These then rapidly rearrange into syn-propanethial-S-oxide, a volatile gas that wafts upward toward your eyes.

This gas reacts with the water in your tear film to form sulfuric acid, which irritates your eyes. Your brain responds by signaling your tear glands to produce more liquid to flush out the irritant - hence the tears. Interestingly, different onion varieties contain varying amounts of these compounds, with sweeter onions generally causing less eye irritation.

To minimize crying while chopping onions:
1. Chill onions first - Refrigerate for 30 minutes or freeze for 10 minutes before cutting. Cold temperatures slow down the enzyme activity that creates the irritating compounds.

2.Use sharp knives - A razor-sharp blade causes less cell damage than a dull one, releasing fewer irritants. Clean your knife frequently while cutting.

3.Work near running water - The gas dissolves in water, so cutting near a running tap or under a vented hood helps pull away the vapors. Some chefs keep a small fan on their work surface.

4.Wear protective gear - Swim goggles or lab glasses create a physical barrier. Even regular eyeglasses offer some protection.

5.Try specialized cutting methods - Leave the root end intact until last (it contains higher concentrations of the enzymes), or cut onions vertically rather than horizontally.

6.Use vinegar - Rubbing your cutting board with vinegar or lemon juice can help neutralize some of the compounds.

Interestingly, cooking onions immediately after cutting can help too, as heat inactivates the enzymes. Some professional kitchens use food processors with closed containers or specialized onion-chopping devices that contain the vapors.

Remember that tearing up while cutting onions is completely normal and doesn't indicate any eye health problems. With these techniques, you can significantly reduce - though perhaps not completely eliminate - the crying effect that has frustrated cooks for centuries.