Richard Feynman loved a good puzzle. In his 1985 memoir Surely You’re Joking, Mr. Feynman!, he described a thought experiment that would bedevil physicists for decades: if you submerge an S-shaped lawn sprinkler and suck water in instead of spraying it out, which way does it rotate?
The answer, it turns out, is that it does rotate, but about 50 times slower than in forward mode. And the mechanism driving it, a new study confirms, is angular momentum flux from centrifugal flows, a principle that applies to sprinklers of any shape, including the whimsically curved “silly sprinklers” found in gardens worldwide.
The puzzle’s long history
The reverse sprinkler problem actually predates Feynman. Ernst Mach experimented with analogous fluid aspiration devices in the 1880s. But Feynman’s popularization of the puzzle, and his inability to settle it definitively, turned it into a fixture of open problems in fluid mechanics.
From the 1980s onward, experiments gave conflicting results. Some showed steady reverse rotation, some showed transient rotation only, some showed no rotation, and some showed rotation direction depending on experimental geometry. The problem acquired the status of a textbook physics paradox.
The 2024 breakthrough
In January 2024, a team led by Leif Ristroph at New York University’s Courant Institute published a landmark paper in Physical Review Letters. They built a custom S-shaped sprinkler with an ultra-low-friction rotary bearing, a floating hub in a water tank, and used laser-illuminated microparticles and dye to visualize the flow.
The result: the reverse sprinkler does rotate, but about 50 times slower than the forward mode. The mechanism is what they called an “inside-out rocket.” Water jets collide inside the central chamber, but because they do not meet exactly head-on, a subtle net torque drives rotation. The driver is centrifugal flow in the curved arms generating angular momentum flux.
The 2026 extension
Now the same group, with new co-authors Jesse Etan Smith and Will Kuhlke joining Ristroph, Mingxuan Zuo, and Brennan Sprinkle, has extended the analysis. Published in PNAS on July 13, the new study tests the momentum flux theory against sprinklers of varied geometries: spirals, loops, complex curves, and commercial “silly sprinklers.”
The theory held across all shapes.
The study also definitively ruled out two competing explanations. Mach’s swirl counter-rotation theory (from the 1880s) could not explain the observed torques. And Feynman’s own outer-arm flow theory, the idea that flows at the outer portions of arms drive motion, was also disproven: outer arm flows had no effect.
“The geometry of the arms governs mass-to-momentum flux conversion,” the authors wrote. Isotropic fluid enters from the far field, swirls up in the curved arms to generate angular momentum, and a residual portion injected inside drives rotation. In forward mode this produces fast rotation; in reverse mode the same mechanism acts in reverse, producing the same direction of rotation but much slower.
Why it matters
Beyond resolving a long-standing physics puzzle, the work has practical implications. Understanding how geometry controls momentum flux in rotating fluid systems can guide the design of turbines, hydrokinetic energy harvesters, and any device that converts fluid flows into mechanical motion.
“Our findings provide a firmer understanding of how components respond to fluid flows, knowledge that can guide future engineering and technological advances for devices, such as turbines, that convert these flows into energy,” said Brennan Sprinkle of the Colorado School of Mines.
And for the garden-variety “silly sprinkler”, those colorful plastic loops and spirals that children run through on summer lawns, the physics, it turns out, is the same.
Sources
Smith JE, Zuo M, Kuhlke W, Sprinkle B, Ristroph L. “Geometry controls momentum flux in the sprinkler problem.” Proceedings of the National Academy of Sciences (2026). DOI: 10.1073/pnas.2537479123
Wang K, Sprinkle B, Zuo M, Ristroph L. “Centrifugal Flows Drive Reverse Rotation of Feynman’s Sprinkler.” Physical Review Letters 132, 044003 (2024). DOI: 10.1103/PhysRevLett.132.044003
Ouellette J. “Solution to Feynman’s reverse sprinkler puzzle also applies to ‘silly sprinklers.'” Ars Technica (July 13, 2026). https://arstechnica.com/science/2026/07/solution-to-feynmans-reverse-sprinkler-puzzle-also-applies-to-silly-sprinklers/