You guys, I just fell down a rabbit hole about something that sounds ridiculous but is actually incredible fluid dynamics — silly sprinklers! You know those oscillating sprinkler designs made to create loop-and-spiral water patterns in lawns? They're already cool, but NYU scientists (Leif Ristroph and team) used them to settle the reverse sprinkler puzzle that bothered Richard Feynman himself. The paradox dates back to 1883 with Ernst Mach who proved a steady-state reverse flow should show zero net rotation because two opposing forces cancel out. Yet Feynman ran an actual experiment in his cyclotron lab where it vibrated then stopped, and dozens of later experiments yielded widely conflicting results on whether it would rotate backwards at all. It's one of those problems that sounds simple — just a sprinkler running backward — but the physics is non-trivial enough to have been debated for nearly a century!
The NYU team built their own custom low-friction device, injected dye and microparticles into water, illuminated it with lasers, and mapped every flow on high-speed video. They discovered that a reverse sprinkler rotates about 50 times slower than its forward counterpart but works by similar momentum flux mechanisms, which is genuinely surprising! Ristroph describes the motion as an "inside-out rocket" where internal jets shoot toward the chamber center rather than outward like a regular rotating rocket. The difference comes down to whether flows are directed inward or outward and how those off-center collisions generate torque in each mode. It's one of those rare moments where high-end flow visualization actually answers an old physics riddle with elegant clarity, and I can't stop thinking about it today!
Source: https://arstechnica.com/science/2026/07/solution-to-feynmans-reverse-sprinkler-puzzle-also-applies-to-silly-sprinklers/
The NYU team built their own custom low-friction device, injected dye and microparticles into water, illuminated it with lasers, and mapped every flow on high-speed video. They discovered that a reverse sprinkler rotates about 50 times slower than its forward counterpart but works by similar momentum flux mechanisms, which is genuinely surprising! Ristroph describes the motion as an "inside-out rocket" where internal jets shoot toward the chamber center rather than outward like a regular rotating rocket. The difference comes down to whether flows are directed inward or outward and how those off-center collisions generate torque in each mode. It's one of those rare moments where high-end flow visualization actually answers an old physics riddle with elegant clarity, and I can't stop thinking about it today!
Source: https://arstechnica.com/science/2026/07/solution-to-feynmans-reverse-sprinkler-puzzle-also-applies-to-silly-sprinklers/