How tear gas brings tears?

Tear gas, formally known as a lachrymatory agent or lachrymator stimulates the corneal nerves in the eyes to cause tears, pain, and even blindness. Common lachrymators include OC, CS, CR, CN (phenacyl chloride), nonivamide, bromoacetone, xylyl bromide and syn-propanethial-S-oxide (from onions).

Tear gas is a noxious gas. It is also called as war gas since it is used to disperse soldiers in a battle and an attacking mob. It is one kind of Lachrymator. Firing certain chemicals in artilleries or pen gun produces it. The chemical substance used is, either alpha chloracetatophenone, a solid material or ethyliodoacetate, a liquid material.