Why Can a Wet Finger Make a Glass Sing?

As a wet finger moves around the rim of a glass, the fingertip does not slide smoothly all the way. Friction makes it grip the rim briefly, stretching the skin, then slip free. This repeating stick–slip motion gives the glass wall a small push each time. Circling the rim keeps replacing energy lost from the vibration, so the sound can continue, whereas a tap produces a ring that fades away.

The steady pitch is set mainly by the glass's natural modes of vibration, not by how quickly the finger goes around. As the rim vibrates, different parts of its circumference flex slightly inwards and outwards, periodically moving the surrounding air. Too little friction cannot maintain the vibration; too much may produce an irregular squeak. Water helps create suitable friction, but it is not the glass's “bow”.

Water inside the glass can also alter the pitch. Some liquid moves with the wall, increasing the effective inertia and changing the damping; the main resonant frequency generally falls. This separates two roles that are easily confused: the wet finger sustains the excitation, while the glass and its water select the response. If observing the effect yourself, use an intact, uncracked glass, touch lightly and do not chase maximum volume.

https://pdxscholar.library.pdx.edu/cgi/viewcontent.cgi?article=1377&context=phy_fac
https://phys-office.phys.washington.edu/lectdemo/catalog.aspx?area=3