Onions contain a chemical with sulphur. When you chop an onion, this chemical is changed into a stronger substance, which is released into the air.
When this vapour reaches your eye, it mixes with the moisture in your tears and forms a type of sulphuric acid, and stings your eye.
In an attempt to dilute this acid, your eye will produce extra tears to wash it out.
Some suggestions for stopping the reflex tears are
- Put the onion in your freezer for 10 minutes before you chop it
- Cut the onion underwater so the sulphuric acid is formed there, instead of in your eye.
- Wear swimming goggles or a diving mask
Not sure if these work… but if all else fails, you could always go out for dinner!!!