There must be some, like vibrations or * * * sounds or something. I have done experiments, only the glass can make sound, and if you hold the glass in your other hand, there will be no sound. The pitch of the sound will also change with the amount of water in the cup ~ It will be very unsmooth to rub with your fingers on the rim of the cup at first, and it will get better when you touch some water on the rim of the cup with your hands ~