Tears are secretions that clean and lubricate the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

s. Lacrimation or lachrymation (from Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 lacrima "tear") is the production or shedding of tears.

Strong emotions such as sorrow
Sorrow (emotion)
Sorrow is an emotion, feeling, or sentiment. Sorrow 'is more "intense" than sadness...it implies a long term state'. At the same time 'sorrow - but not unhappiness - suggests a degree of resignation...which lends sorrow its peculiar air of dignity'....

 or elation
Happiness is a mental state of well-being characterized by positive emotions ranging from contentment to intense joy. A variety of biological, psychological, religious, and philosophical approaches have striven to define happiness and identify its sources....

, irritation of the eyes, and yawning may lead to an increased production of tears, or crying
Crying is shedding tears as a response to an emotional state in humans. The act of crying has been defined as "a complex secretomotor phenomenon characterized by the shedding of tears from the lacrimal apparatus, without any irritation of the ocular structures"...

In humans, the tear film coating the eye, known as the precorneal film, has three distinct layers, from the most outer surface:

Having a thin tear film may prevent one's ability to wear contact lenses, as the amount of oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 needed is higher than normal, and contact lenses stop oxygen from entering the eye.