Imagine a sprinkler that shoots water inward instead of outward—and somehow spins anyway. For over a century, scientists have puzzled over how this reverse sprinkler works, and now a team of mathematicians may have finally cracked the case using specially designed "silly sprinklers" with loops and spirals.
The mystery dates back to the 1980s, when physicist Richard Feynman famously tried and failed to build a reverse sprinkler in his lab. The question seemed simple: if water flows into a sprinkler instead of spraying out, which way should it turn? Scientists proposed different explanations, but nobody could prove which one was right.
Leif Ristroph and his team at New York York University's Courant Institute recently took on the challenge. Rather than testing ordinary sprinklers, they built their own quirky versions—shaped with loops, curves, and spirals—which let them measure exactly how the water moved and how much force it created.
Their key finding? The reverse sprinkler spins about 50 times slower than a normal one, but it definitely does spin. The researchers discovered that two jets of water inside the sprinkler collide—but not perfectly head-on. That tiny misalignment creates just enough force to make it rotate in the opposite direction from a regular sprinkler.
"This work provides the experimental answer for Feynman's Sprinkler Problem by showing, across several sprinkler types, how the angular momentum of water flows drives sprinklers' rotation," Ristroph explained.
The team tested their theory against two older explanations. One, from physicist Ernst Mach in the 1880s, suggested the fluid swirls in one direction while the sprinkler turns the other way. But that theory couldn't explain the reverse rotations the researchers observed. Another idea, linked to Feynman himself, said the water flows on the outside of the sprinkler arms were the key. The experiments, however, showed those outer flows had no effect at all on how the sprinkler moved.
So what does solving a 140-year-old physics puzzle actually matter? According to co-author Brennan Sprinkle of the Colorado School of Mines, the knowledge could help engineers design better devices that convert fluid flows into usable energy—like windmills or water turbines.
"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," Sprinkle said.
The research was published in the journal Proceedings of the National Academy of Sciences. So the next time you see one of those whimsical sprinklers shooting water in circles, you might appreciate them a little more—they just helped scientists untangle a piece of how the physical world works.