Brain

1. Volunteer
Stimulates: The prefrontal cortex, which analyzes, plans, and problem-solves
Why: A Johns Hopkins study found that older women who tutored kids for six months developed sharper cognitive skills. The social and mental activity required for teaching sends blood rushing to this part of the brain.

2. Work out
Stimulates: The hippocampus, which forms memories
Why: Arthur Kramer, PhD, a researcher at the University of Illinois, used MRIs to show that exercise actually makes your hippocampus bigger. Physical activity may increase the number of capillaries in the region, which in turn helps new cells grow. Kramer prescribes one-hour sweat sessions three times a week.

3. Learn a skill
Stimulates: The intraparietal sulcus, which directs hand-eye coordination
Why: At Oxford University, researchers taught 24 people to juggle and found that after six weeks this region had a higher density of white matter (the fibers that let neurons communicate). Any novel activity that is practiced intently, such as tennis or guitar playing, will likely have this effect, says study author Heidi Johansen-Berg.

Next: Why wiggling your eyes can save your memory