You are enjoying a glass of nice wine with your date. You observe the color, smell the wine, swirl your glass, and then take a sip of the wine. Now check out the surface on the inside of the glass. You would notice a thin film of wine, which will drip down the sides of the glass back to the surface of the wine. The descending film or little droplets are referred as wine “tears” or “legs.” (Some would even call it "curtains" or "church windows".)
Some wine lovers religiously observe the wine "tears". Many argue that the more tears, the higher the wine quality. However, you should know that the occurrence of these tears is simply a manifestation of a physics phenomenon - an interplay between adhesion, evaporation, water's surface tension and alcohol content. It has nothing to do with the perceived quality or age of the wine. The phenomenon is known as the Marangoni Effect.
For those who are not interested to understand the details, you can skip the following two paragraphs. But it's actually quite interesting, something that we all learned back in physics and chemistry class in high school.
Wine tears are formed because ethanol (or alcohol) evaporates more quickly and is more non-polar than water. Ethanol is also surface-active in wine; this results in a higher concentration of ethanol molecules than water molecules at the surface of wine. Ethanol’s ability to evaporate more quickly than water, particularly at the thinnest part of the film (on the sides of the glass after you swirl it), will leave behind more "watery" wine in that area.
Water, in general, has a high "surface tension"; basically water likes to cling tightly to other water molecules (that's right, this is due to Hydrogen-Bond, thus the polar nature of water molecules.) Ethanol has a lower surface tension than water, as it is more non-polar. A mixture of water and ethanol (i.e. wine) has a lower surface tension than a solution of pure water. Thus, after ethanol evaporates from the surface and the ethanol concentration decreases, the remaining water molecules increase the surface tension of the wine, creating a surface tension "gradient" (difference in degree of surface tension between two bodies). The wine left in the bulk liquid responds to this gradient by climbing toward the top of the film to reduce the surface tension. This forms a ring around the wine glass above the main surface of the wine, and the ring grows as this process repeats. Eventually, gravity pulls the ring back into the wine, creating what we refer to as tears or legs. This phenomenon was named after Italian physicist Carlo Marangoni in 1865.
The amount of "tears" simply indicates the amount of alcohol content in the wine. Any wine with >11-12% alcohol is likely to demonstrate quite good legs. Unless you are contrasting a light wine with a fortified wine, the quality or amount of the "legs" is not a tremendously useful indicator of the alcohol content either for regular table wines.
Here's a fun experiment you can show your wine buddies at the next gathering. Try covering your glass of wine (e.g. with plastic shrink wrap) and do your usual cool swirling move. You would see that the legs present dramatically decrease when covered compared to when open. When covered, you essentially stop the alcohol evaporation, thus no legs would form.
So next time when your friends are observing the "legs", you can pull out this term called Marangoni Effect. You can even try this experiement to show off your knowledge in physics. Of course, this will also show your geeky nature hidden inside!