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Run her backwards, sucking water in, and she still turns.\u003c\/p\u003e\u003ch3\u003eThe Physics\u003c\/h3\u003e\u003cp\u003eA sprinkler that sprays water spins because of the jets' reaction. Run it in reverse underwater, sucking water in, and it turns the other way, unsteadily and about 40 times slower than forward at the same flow rate. The curved arms fling the inflow outward, so the two jets meeting in the hub miss head-on and push it round.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eτ = ρ Q v r   (forward, held still)\u003c\/strong\u003e\u003c\/p\u003e\u003cul\u003e\n\u003cli\u003eτ: stall torque (N·m)\u003c\/li\u003e\n\u003cli\u003eρ: fluid density (kg\/m³)\u003c\/li\u003e\n\u003cli\u003eQ: volume flow rate (m³\/s)\u003c\/li\u003e\n\u003cli\u003ev: jet exit speed (m\/s)\u003c\/li\u003e\n\u003cli\u003er: arm radius (m)\u003c\/li\u003e\n\u003c\/ul\u003e\u003cp\u003eMach posed it in 1883, Feynman popularised it, and it was settled experimentally in 2024 by K. Wang, B. 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A wave fits around the circle when a whole number of wavelengths matches the circumference. Light does the same in tiny glass rings and spheres.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003e2πR ≈ m λ\u003c\/strong\u003e\u003c\/p\u003e\u003cul\u003e\n\u003cli\u003eR: radius (m)\u003c\/li\u003e\n\u003cli\u003em: whole number of wavelengths (dimensionless)\u003c\/li\u003e\n\u003cli\u003eλ: wavelength (m)\u003c\/li\u003e\n\u003c\/ul\u003e\u003cp\u003eLord Rayleigh explained the whispering gallery of St Paul's Cathedral in 1910; optical whispering-gallery resonators reach Q \u0026gt; 10¹¹.\u003c\/p\u003e\u003cp\u003e\u003cem\u003eHonest caveat:\u003c\/em\u003e It is not an echo: the sound hugs the wall.\u003c\/p\u003e\u003ch3\u003eOn the shirt\u003c\/h3\u003e\u003cp\u003eFront: Murmura, the pixel-art character, with the title. Back: the resonance condition, the St Paul's story, the optical resonators and Rayleigh's 1910 paper. 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