The tendency of molecules of the same kind to stick together is called cohesion, and the tendency of molecules of different kinds to stick together is called adhesion. For example, water is the most cohesive nonmetallic substance because its hydrogen and oxygen atoms attract each other.
The tendency of water molecules to clump together is evident in the teardrop shape of rain and in the circular droplets of isolated water on a surface. Adhesion is caused by several forces, including electrostatic, mechanical and gravitational. Water poured on glass is adhesive and spread uniformly over the glass because water has a greater affinity for the glass than for other water molecules.