Water is a polar molecule, composed of two hydrogen atoms bonded to a single oxygen atom (H2O). Water molecules like to stick to one another, like small magnets. This is called cohesion. Water molecules also can be attracted to other substances, such as metal or dirt. This is called adhesion. This picture was taken during the rinse cycle at the car wash today. All of the dirt and salt is gone from my windshield so cohesion reigns and the water beads up. And you thought this was just a photo blog. Photo made with a Canon G10, 1/200s, f/4.0, ISO 100, 59mm equivalent focal length in macro mode, mounted on the dash using a Monsterpod, and shutter fried with a Zap Shot remote.