Every once in a while you will see someone swirl their glass of wine, raise it towards the light and watch with bated breath for the wine's legs to appear, a mythical indicator of wine quality. These legs, or tears as the French refer to them, are the streaks of wine forming on the side of the wine glass. The legs were once thought to be associated with a wine's quality (the more legs, the higher the quality). However, the legs have more to do with physics, the wine's surface tension and alcohol content, than perceived quality.
Wine is a mixture of alcohol and water, the alcohol has a faster evaporation rate and a lower surface tension than water, effectively forcing the alcohol to evaporate at a faster rate. This dynamic allows the water's surface tension and concentration to increase, pushing the legs up the glass until the surface tension pushes the water into beads. Finally, gravity wins the battle and forces the liquid to tear down the glass in a defeated streak. Still not convinced that it's physics and not quality that drives this phenomenon? Try covering your next glass of wine and see if the legs present dramatically decrease when covered compared to when open. No evaporation, no legs. Enjoy none the less.