The brain, just like a muscle, needs to work out to stay fit. By challenging and pushing the brain to reach higher and do more, you make it stronger. Studies have shown that animals put into an enriched environment with many opportunities for exploratory activity actually undergo structural changes in the brain, improving their abilities both to learn and to remember, says Molly V. Wagster, Ph.D., program director of the neuropsychology of aging program at the National Institute on Aging in Bethesda, Maryland.

But what may be a challenge one day may be easy going the next. Although mastering any new mental task gets more difficult as you get older (no one knows why), once you have completely mastered a task, no matter how difficult, performing it no longer improves your brain. "We tend to reach plateaus in life. We work at something until we are good at it, then we continue to do it at the same level. But as you continue, the challenge disappears," says Arnold Scheibel, M.D., professor of neurobiology and

psychology and former director of the Brain Research Institute at the University of California, Los Angeles.

In order to keep your brain and memory healthy as you get older, you must constantly seek out new challenges for them. Here are some recommended by Dr. Scheibel that will really give them a workout.

Take a language class.Learning a new language especially targets those areas of the brain that enhance memory. "It is a wonderful stimulus to memory function," Dr. Scheibel explains. If you don't have time to take a class, try learning from audiotapes. Use them as you drive to work each day. "Just work on it a half hour a day. You can make a lot of progress in a couple weeks," he says.

Make beautiful music. The combination of visual, mental, and physical skills you use playing an instrument invigorates your brain, he says. Music, like many creative pursuits, improves your memory and provides a wonderful expressive outlet.

Seek out the opposite. Work on skills that are the exact opposite of what comes naturally to you. If you are artistic, dabble in the logical world of math or computers. If you are verbal, test out your ability to express yourself visually in painting or drawing.

Calculate in your head. Dr. Scheibel refuses to use calculators or computers to do basic math. Why? Because he found that relying on these timesaving devices soon eroded his ability to do math in his head. Math calculations challenge your brain and keep your memory in good shape.

Switch hands. For about 5 minutes a day, try being a lefty if you are a righty, or vice versa. Write, punch numbers into the telephone, or play tennis with your nondominant hand. This exercise develops the opposite side of your brain.

Surround yourself with interesting people. Dr. Scheibel compares keeping company with intellectually stimulating people to playing tennis. "The very best thing you can do is play with someone better than you because you strive to be better. It is the same thing with people. When you feel, 'Gee, I am just a little bit out of my class here because everyone seems so much brighter,' that makes you extend, and I can't think of a better challenge," he says. Join book clubs, take classes, form discussion and writing groups, or just hang out with people who challenge you in conversation.