When the garlic organization is crushed, it can release garlic glycosides enzyme, which can change these organic molecules into time sulfonic acid time sulfonic acid which can then immediately and naturally form the chemicals of propane sulfur aldehyde and sulfur oxide which can cause the tears. Rich nerve endings can find that corneal contacting to the conjunction of propane sulfur aldehyde and sulfur oxide can cause ciliary nerve activity which spreads to the eyes. That is whole reason why the eyes tear when using the garlic. If you want to avoid such thing happen, you could add some water to prevent its volatility when using the garlic. Such easy thing is suggested by a lot of experienced people.
Garlic, as well as onion and shallot, belongs to allium in botany. All allium plants contain a chemical substance called allinase. Garlic contains 20% of volatile grease whose main element is allinase. The allinase is not volatile itself. Only when garlic is suffering mechanical damage such as being crushed, it will give off the allinase of spicy smell. Allinase will turn into sulfenic acid in air and sulfenic acid will reorganize as syn-propanethial-S-Oxide (SPSO). It is SPSO that arouses watery eyes. Cornea, with a lot of nerve fibers on it, is the first layer of protection of human eyes. When SPSO in air reaches cornea, it will give stimulation to nerve fiber and nerve fiber will send the stimulation to brain. Brain reads the stimulation as burning pain and give out a signal to rinse it off to tear gland. Tear gland begins to secrete tear and eye become watery.
Garlic will release garlic glycosidase, it can transform organic molecules into acid and form the chemicals which can cause tears such as thio aldehyde and sulfur oxides.Neural activity stimulate the autonomic nerve fibers, autonomic nerve fibers turn the signal back to the eyes, then tears can wash away the irritating substance.
If you have tears, you can open the refrigerator freezer and put your head in for a few seconds, then your eye will don't flow.