To cut your risk of Alzheimer's by more than half, just do this for 20 minutes twice a week: walk.

Exercising for 20 to 30 minutes a couple of times a week not only slashed Alzheimer's risk by as much as 60 percent in a study but also cut the risk of regular dementia in half.

Lifestyle vs. Genes
In the study, midlife exercise appeared to be key in warding off mental decline later in life. And those who had genes that made them more susceptible to Alzheimer's reaped the greatest protective benefits from physical activity. Great to know that genes don't necessarily control your destiny, right?

Little Effort, Big Rewards
People in the study didn't have to exercise hard to protect their brain, either. A couple of moderately intense workouts a week was all it took. Researchers suspect that exercise protects the brain by promoting blood vessel health and boosting the brain's ability to repair damage. Here are three more great ways to keep your noggin young: