So I’m getting older. It’s true, for now, and much better than the alternative. As part of getting older I gain weight faster, my joints creak more, and apparently my brain shrinks.
If you’re around my age or older you know this is true, and if you’re younger — ha ha! It’s gonna happen to you, too.

If you want to keep your brain from shrinking, put yourself in a glass box and run in place.
But if you want to avoid all this there’s one easy thing to do. Get yourself a terrarium, fill it full of shiny objects and balls and bells, and get an exercise wheel. Oh, and you can leave out the shiny objects and balls and bells.
The most persuasive evidence comes from several new studies of lab animals living in busy, exciting cages. It has long been known that so-called “enriched” environments — homes filled with toys and engaging, novel tasks — lead to improvements in the brainpower of lab animals. In most instances, such environmental enrichment also includes a running wheel, because mice and rats generally enjoy running. Until recently, there was little research done to tease out the particular effects of running versus those of playing with new toys or engaging the mind in other ways that don’t increase the heart rate. …
“Only one thing had mattered,” Rhodes says, “and that’s whether they had a running wheel.” Animals that exercised, whether or not they had any other enrichments in their cages, had healthier brains and performed significantly better on cognitive tests than the other mice. Animals that didn’t run, no matter how enriched their world was otherwise, did not improve their brainpower in the complex, lasting ways that Rhodes’s team was studying. “They loved the toys,” Rhodes says, and the mice rarely ventured into the empty, quieter portions of their cages. But unless they also exercised, they did not become smarter.
via Novelist Meg Clayton