Silly Sprinklers Help Scientists Finally Solve Feynman's 60-Year-Old Sprinkler Mystery

Every summer, lawns fill with colorful "silly sprinklers" whose looping tubes send water spraying in unusual patterns. Their designs may look playful, but researchers have now used these backyard devices to investigate a serious and decades-old question in physics known as Feynman's Sprinkler Problem.

The problem asks what happens when a sprinkler operates in reverse, pulling water into its arms instead of forcing water outward. Nobel laureate Richard Feynman famously posed the question, and generations of physicists have debated the answer without reaching a consensus. By building and testing sprinklers with a variety of shapes, a team of mathematicians from New York University and the Colorado School of Mines has now produced a clear experimental answer.

Three things worth knowing:

The team's experiments showed that the rotation of both normal and reverse sprinklers is driven by the momentum of flowing water, not by the outside water flow. In forward mode, water exiting the arms pushes the sprinkler around. In reverse mode, water entering the arms also creates a rotational force, but in the same direction as the forward mode — just much weaker, about 30 times slower.

The findings, published in Proceedings of the National Academy of Sciences, settle a puzzle that has persisted since the 1940s. Feynman himself had built a homemade reverse sprinkler in his bathtub but never published a definitive result. The mystery lived on in physics textbooks, classroom debates, and even scientific papers that offered conflicting theoretical predictions.

Beyond solving a famous puzzle, the research has practical value. Understanding how objects react to moving fluids could help engineers improve turbines, hydroelectric generators, and any device that converts the energy of flowing liquids into useful work. The same principle that makes a reverse sprinkler rotate slowly is relevant to designing efficient fluid-energy capture systems.