Drag the dial round. A spin-½ wave is split: one half stays, one half is turned by θ. You hear them recombined. Turn one full lap (360°) and the bell goes silent: the turned half came back with its sign flipped. Only a second lap (720°) brings the sound home.
LIT a 2π rotation multiplies a spin-½ state by −1; seen in neutron interferometry (Rauch et al. 1975; Werner et al. 1975). MODEL this is Dave's 720° clock. ANALOGY neutron phase played as sound.
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