Researchers have established that when people are mentally (1) , biochemical changes occur in the brain that allow it to (2) more effectively in cognitive (认知的) areas such as attention and memory. This is true (3) age. People will be alert and (4) if they are faced with information that (5) them to think about things they are (6) in. And someone with a history of doing more (7) less will go into old age more cognitively sound than someone who has not had an active (8) Many experts are so convinced (9) the benefits of challenging the brain that they are (10) the theory to work in their own lives. "The idea is not (11) to learn to memorize enormous (12) of information," says James Fozard, associate director of the National Institute on Aging. "Most of us don’t need that kind of skill. Such specific training is (13) less interest than being able to maintain mental (14) ." Fozard and others say they challenge their brains with different mental skills, (15) because they enjoy them and because they are sure that their range of activities will help the (16) their brains work. Gene Cohen, acting director of the same institute, says that we are frequently advised to keep (17) active as we age, but older people need to keep mentally active as well. Those who (18) are more (19) to maintain their intellectual abilities and to be generally happier and better adjusted. And intellectual activity actually (20) brain-cell health and size.