In a quiet corner of the Dutch countryside, a spinning wheel placed in the open air became an unexpected window into animal nature. Researchers from Leiden University discovered that wild mice, rats, frogs, and even slugs chose to use exercise wheels entirely on their own terms — not because captivity had broken them, but because movement itself appears to carry an intrinsic reward. Over three years and more than 200,000 recorded visits, the experiment gently dismantled a long-held assumption: that wheel running was a symptom of confinement rather than an expression of something genuinely aliv