250px|thumb|The tear system. A) Tear gland / Lacrimal glandB) Superior lacrimal punctumC) Superior lacrimal canalD) Tear sac / Lacrimal sacE) Inferior lacrimal punctumF) Inferior lacrimal canalG) Nasolacrimal canal Tears are a liquid process of lacrimation to clean and lubricate the eyes. The word lacrimation may also be used in a medical or literary sense to refer to crying. Strong emotions, such as sorrow or elation, may lead to crying. Although most land mammals have a lacrimation system to keep their eyes wet, Humans are the only animal generally accepted to cry emotional tears. 
In humans, the tear film coating the eye has three distinct layers, from the most outer surface:
The lipid layer contains oils secreted by the meibomian glands. The outer-most layer of the tear film coats the aqueous layer to provide a hydrophobic barrier that retards evaporation and prevents tears spilling onto the cheek.
The aqueous layer contains water and other substances such as