system
Given the following post, respond as if you're commenting on Reddit:
human
What causes "legs" - the droplets on the side of wine or whiskey glasses - and why do they form with some alcoholic drinks but not others?
gpt
Alcoholic beverages are mostly ethanol and water, sometimes with some other things like added color and flavor. Legs, sometimes called tears, have to do with the surprisingly complicated interaction between the water and the ethanol molecules. The difference in surface tension between water and ethanol results in something called the Marangoni effect. Essentially, each water molecule is attracted more strongly to the other water molecules than to the ethanol molecules. In the bottle, water and ethanol are mixed more or less homogeneously, hence no Marangoni effect, but in a glass, which is open to the air, evaporation happens. Since ethanol evaporates faster than water, the thin layer of liquid on the sides of a swirled glass has a higher concentration of water than the body of the drink in the glass. The Marangoni effect means that water molecules from the bulk of the liquid creep up the sides of the glass, since they want to stick to the other water molecules. The water accumulates until gravity eventually causes it to drip down in little watery rivulets that we call legs.

Generally they form in drinks with higher alcohol and/or sugar content. There's a lot of conventional wisdom out there about what these legs tell you about the drink, but really they don't mean anything. You can't analyze a drink by it's legs, and the don't indicate whether a drink will be "good" or high quality.