Meniscus is the curvature of a liquid's surface within a container such as a graduated cylinder. However, before we explain why some liquid have a concave up meniscus while others share a concave down meniscus, first we have to understand the adhesive forces at work on surface tension. Water, for example, is a polar molecule that consists of a partial positive charge on the hydrogens and a partial negative charge on the oxygen. Thus, within liquid water, each molecule's partial positive charge is attracted to its neighbor's partial negative charge. This is the origin of the cohesive forces within water. Water molecules buried inside the liquid is then being pulled and pushed evenly in every direction, producing no net pull. Meanwhile, the molecules on the surface of the liquid, lacking pulling forces in the upward direction thus encompass a net downward pull.