How challenging do mental exercises need to be to benefit my brain?

One way to keep your brain strong is by, as researchers call it, "testing at your threshold." One large-scale project measured to see whether testing at your threshold was able to reverse and cause new growth of neurons and dendrites (the part of the neurons that catches information from neurotransmitters).

In the project, a computer was programmed to understand a subject's math ability. It then programmed a test so that each person stayed in line with his or her ability. Once the computer pushed the limit of each person's ability-testing at his or her threshold-researchers were able to see growth of neurons and dendrites (as judged by imaging scans of the subjects' brains).

But the best part? The people didn't need to get the answers right in order to reap the benefits. Simply testing themselves just slightly beyond their capability (80 percent correct and 20 percent wrong answers) was enough to cause the re-growth.

So for you, let's say you can always do Wednesday's crossword puzzle but barely get half of Sunday's answers. The best thing for your ego may be for you to continue to master Wednesday's, but the best thing for your brain is to keep taking a whack at Sunday's (as long as it's not so frustrating that there's no fun in doing it).

Just like an athlete becomes faster or stronger by training to attain goals that are just out of reach, you can train your brain to stay smarter and sharper.

YOU: The Owner's Manual, Updated and Expanded Edition: An Insider's Guide to the Body that Will Make You Healthier and Younger