YOU NEED:

Glass of port (or other strong wine) belonging to a parent (or any other over-18 relative) 

METHOD:

Swill the port around the edge of the glass slightly, watch closely and wait…

RESULT:

You should notice that drops of port keep slowly falling down the sides of the glass, even after you’d think that all the port that had swilled up onto the sides had fallen back in. The port just keeps coming! 

The reason that port keeps raining down the sides (seemingly from nowhere) is that the surface of a liquid acts like a stretched out rubber sheet, which pulls objects on the surface equally in all directions. How strong the pull is depends on the particular liquid. Water for example, pulls things stronger than port does.  Port contains a mixture of water and alcohol. When you swill the port onto the edge of the glass, the alcohol evaporates away from the sides, leaving a thin layer of water. Since this thin layer of water acts like a stretched out rubber sheet, it pulls on the port in the glass below, bringing it up the side of the glass. When enough port is collected near the top of water layer it forms drops that are too big to be held up any more, and they fall back down into the glass. So the layer of water on the edge acts like an elevator, lifting more and more port up the side of the glass, so that it can keep dripping back down.