It's water. Ethanol in water (mixture) severely reduces the surface
tension of water, meaning water cannot 'bead'. However when this same
mixture finds itself on the side wall or neck of a glass bottle, it
becomes exposed to the air trapped in the neck, and the ethanol
content evaporates, leaving behind the water fraction. The water then
regains it's physical property of surface tension, and sticks
or 'beads' on the glass.
This is the same process that forms the 'tears' or 'legs' you see on
the side of a glass of wine or spirits. The only difference there is
that because the glass is open to atmosphere, more and more ethanol
will evaporate, leaving behind more water, which collects in heavy
drops like raindrops on a window, and trickle back down into the
Thanx Harry, clear and concise, appreciate it.