Legs and tears on the glass

Swirling a glass and studying the streaks is the most widely performed piece of wine theatre there is, and it is almost entirely uninformative.

The phenomenon has a name and a well-understood physics: it is the Marangoni effect, described in the nineteenth century and observable in any alcoholic drink.

Swirling leaves a thin film of wine on the glass above the surface. Alcohol evaporates from that film faster than water does.

The film is left with a higher proportion of water than the wine below it, and therefore a higher surface tension. That gradient draws liquid up from the bulk of the wine.

Liquid accumulates in a ring at the top of the film until gravity overcomes the effect, and it runs back down as the visible drops.

Higher alcohol concentration produces a steeper gradient and therefore more pronounced legs. Glycerol and sugar add viscosity and can slow the drops, but alcohol dominates.

Non-wine variables matter more than most people realise: a greasy glass, low humidity or a warm room all change the appearance without the wine changing at all.

Mainly as an inoculation. A reader who knows what legs are will not be persuaded by someone using them to make claims about a wine, and will not waste attention on them.