Tap a glass or slide over them. More water makes a lower sound.
Dots are your glasses. The curve shows pitch (Hz) for each water level.
When you tap a glass, its wall rings like a bell. Water pressed against the wall adds mass, so the wall moves more slowly and the sound gets lower. This is why a full glass sounds low and an almost empty glass sounds high.
Simple model: the wall acts like a spring with an extra mass of water.f = f0 / sqrt(1 + a * h)
f0 is the ring of the empty glass (here 1090 Hz), h is the fill fraction (0 to 1) and a (here 16.5) says how much the water weighs compared with the vibrating wall. Real glasses differ, so these numbers are a tuned toy, not a measurement of one particular glass.
Different from blowing over a bottle: then the air column sings, f = v / (4L), and more water means a higher note. Tapping and blowing give opposite trends.
Music: each step of the scale is a frequency ratio. Equal temperament uses f = 440 * 2^((n-69)/12). A glass counts as in tune within 25 cents (a quarter of a semitone).
Check: we simulate a mass on a spring step by step (RK4), measure its period and compare with the formula. We also generate the sound offline and measure its pitch.