Subject: Why does a water film increase friction between some surfaces ?

For example, when dragging a flat bottomed glass  across a kitchen counter 
(marble), adding water drastically increases the force necessary to move the 
glass. When performing the "singing glass" experiment, water provides the 
necessary friction to get the stick-slip motion going. How is that ? I tended to 
think that water would act as a lubricant - albeit a poor one - but not an 
antilubricant ! Thanks in advance for your anwser,

Nicolas   

Re: Why does a water film increase friction between some surfaces ?