A camera looks at a sealed, opaque bottle and sees exactly one thing — a bottle — whether it's empty, half, or full. Tap it, and the pitch tells you instantly. Fill level is a hidden property: light and touch can't reach through the wall, but sound is made of what's inside. Drag the fill and listen to the level become audible.
Two views of the same bottle
The resonant pitch
Guess the fill — from the sound alone
A mystery pour hides the level and plays one tap. Your eyes are useless here; your ears aren't. Slide to your guess, then reveal.
Helmholtz resonance f = (c/2π)·√(A ÷ V·L′) with a shrinking air volume — exact first-principles physics. The struck model f ∝ 1/√(1 + α·fill) is a mass-loading (Rayleigh-quotient) approximation, α set so a full glass rings ~0.6× the empty one. Both Node-verified monotonic and audible. Tones synthesized live on-device. · Institute for Physical AI @ BMI