A mechanism for the rupture of the tear film is identified. The key step in the process of tear film breakup is the instability and eventual rupture of the mucous layer of 0.02 to 0.04 μm thickness, which covers the epithelium. The rupture is caused by the van der Waals dispersion forces acting on the mucous layer. The aqueous part of the tear film then ruptures because it comes into contact with the exposed hydrophobic epithelium. The time of rupture derived from the proposed mechanism is consistent with the range of observed breakup times (BUT's) and other clinically observed characteristics of rupture. The proposed mechanism has implications in understanding the cause of adhesion of the contact lens to cornea and in the rapid buildup of deposits on the lens surface, both of which are attributed to the mucous deficiency. It also suggests various possibilities for diagnosis and treatment of pathological conditions of a dry eye, and lends support to existing ones.