Sprinkla

Sprinkla spins on one toe, arms bent like a lawn sprinkler. Run her backwards, sucking water in, and she still turns.

The Physics

A 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.

τ = ρ Q v r (forward, held still)

• τ: stall torque (N·m)
• ρ: fluid density (kg/m³)
• Q: volume flow rate (m³/s)
• v: jet exit speed (m/s)
• r: arm radius (m)

Mach posed it in 1883, Feynman popularised it, and it was settled experimentally in 2024 by K. Wang, B. Sprinkle, M. Zuo and L. Ristroph (Phys. Rev. Lett. 132, 044003).

Honest caveat: The torque formula is an ideal, tangential-jet idealisation.

On the shirt

Front: Sprinkla, the pixel-art character, with the title. Back: the forward torque, the reverse-rotation result, the hub-jet explanation and the 2024 paper. Printed on a unisex tee.