What is happening when you rub your finger around a glass rim?

Running a finger around the rim of a glass will "stimulate" the glass. We are adding energy to the glass, and because the finger tends to "slip and grab" as it moves around the rim, the energy is delivered in "pulses" to the glass. This tends to make the glass vibrate, and we are adding to the vibrational energy by continuously running our finger around the rim of the glass. The net result is that the glass will "sing" to us in a given pitch.

Some glasses lend themselves better to this experiment than others, and most stemware works quite well, even "cheaper" types. When doing an experiment, moistening the finger will help things. This is a good project to investigate some things in basic physics, though the principles at work may be a bit less than clear. The glass, like other objects, has a natural resonance, and we "tap into" that by mechanically stimulating the glass at the rim with our finger. When we find the glass beginning to vibrate, we can move our finger a bit faster or slower to add energy "in sync" with the vibrations, thus sustaining and increasing them.