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.