Sound is produced by vibration. When you tap a glass, the glass wall and the water inside vibrate together as a single system. The pitch — how high or low the note sounds — is determined by the frequency of that vibration: how many times per second the glass and water move back and forth.

Frequency depends on mass. A system with more mass resists changes in motion — it takes more force to get it moving, and it moves more slowly. A glass with more water has more mass, so it vibrates more slowly, producing fewer cycles per second and a lower pitch. A glass with less water has less mass, vibrates more freely, moves faster, and produces a higher pitch.

This is the same principle that governs every tuned percussion instrument. A xylophone's long, heavy bass bars vibrate slowly for low notes. Its short, light treble bars vibrate quickly for high notes. The glasses in this experiment work exactly the same way — adjusting the water level is equivalent to swapping out bars on a xylophone.

The food colouring plays no role in the sound — it simply makes each glass visually distinct, turning the row into a colour-coded instrument that students can read and play.