The glass harp produces sound via the 'stick-slip' phenomenon. When a wet finger rubs the rim of a crystal glass, it momentarily sticks and then slips. This rapid vibration causes the glass to flex inward and outward, creating a pure longitudinal wave. The pitch is determined by the glass’s thickness and the amount of water inside, which acts as a damping agent. More water increases the mass of the vibrating system, which slows the frequency and lowers the pitch.