Puzzles
Run a lawn sprinkler backward, sucking water in — which way does it spin?
A rotating sprinkler normally spins because water shoots out of its curved arms. Submerge it and make it suck water in instead, reversing the flow — does it spin the same direction, the opposite direction, or not at all?
Reveal the answer
It stays essentially still, turning only very weakly in reverse under specific low-friction conditions — a result debated for over a century. Ernst Mach first posed the problem in 1883; it's popularly linked to Richard Feynman after he described tinkering with it at Princeton. A team finally nailed down the physics with high-speed video in 2024, more than 140 years after Mach's original question.