Too often he has heard the expression "tears of wine". This expression explains the phenomenon that can be observed by shaking a glass of wine so this slip through your walls and wet.
On standing the glass, the wine has been on the wall falls, but does so as if "cry", forming "tears" that slide into the liquid that awaits them below.
The reason why the wine, and other spirits with alcohol content, "cry" is due to the different rate of evaporation of water and alcohol on the inside surface of the glass.
Alcohol, being more volatile, evaporates more quickly than water. This, which is now in greater proportion, tends to meet, forming droplets, as their molecules are joined together more strongly than with alcohol. The wine wall, now depleted in alcohol has evaporated, then meets drops sliding on the inner surface of the glass.
Therefore, we dilute tears this wine in a small proportion. But as in the case of our tears, a simple handkerchief or a napkin, also dry the tears of wine. If we cover the cup with a napkin and waved as we have done above, the tears will not form: alcohol will not evaporate on the cover glass, so that the phenomenon will not happen.