Allow me to explain what I mean... ((I am asking this question in the physics section in hopes of finding a "physics" argument))
I shall make it clear what I mean by demonstrating an experiment...
Hold your hand upright with your index finger pointing up towards the sky. Now drop about 2-3 drops of water right on top of your finger. Water drop should (theoretically) stay there. Now tilt your hand ever so slightly towards one side. The water drop rolls down your finger down your wrist leaving a trail towards the tilted side right?
Now's here comes my question. Now if you bring your finger and the hand to the initial upright state and place another water drop on the same finger. Now we see that the water drop takes the same trail! Why does it happen?
My argument: The water trail which was previously left off reduced the friction in that path by filling the gaps between the molecules. Hence the new drop tends to slide more on that path and hence ends up travelling in it.
((This "effect" is also observed when tear drops keep rolling down in the same path. But that could just be because that path is the steepest.))