Tears are made up of a mixture of components that must be kept in a perfect balance to ensure the eyes remain properly moistened. Every time we blink, a thin layer of tears spreads across the eye's surface. This layer, known as the tear film, forms a protective coat that lubricates the eye and washes away any foreign objects. The tear film is made up of three layers:

1. Aqueous (water) layer €“ the largest component, this is produced by the lacrimal gland on the upper, outer portion of each eye. On its own, this layer would quickly run off the eye's surface or evaporate, but the other layers of the tear film help to keep it in place

2. Lipid layer €“ produced by oil-secreting (meibomian) glands along the edge of the upper and lower eyelids. This is found on top of the aqueous layer and prevents it from evaporating too quickly

3. Mucin layer €“ created by cells on the eyelids and eye surface below the aqueous layer. This coats the eye and allows the aqueous layer to 'stick' to the otherwise water-repellent cornea.