For decades, one of neuroscience's greatest mysteries was this: why do some people's brains show the physical changes linked to dementia and Alzheimer's, while their minds and memory stay perfectly sharp?

A long-term study on Catholic nuns helped resolve this puzzle. When researchers examined their brains after death, some showed the full physical hallmarks of Alzheimer's disease — plaques, tangles, all the classic markers. Yet these women had shown no signs of memory loss while alive.

Something protected them from ever experiencing the effects of that disease. Researchers call it cognitive reserve — a mental buffer, built over a lifetime, that lets some brains keep functioning well despite age-related wear and tear.

Here's the hopeful part: cognitive reserve isn't something you're born with. It's something you build. Researchers estimate that up to 45 percent of dementia cases worldwide may be preventable or delayable through modifiable lifestyle factors.

So what builds it? Not crossword puzzles or brain-training apps, despite popular claims. The stronger research evidence points to something simpler: movement, and yoga in particular.