Can exercising your brain stave off Alzheimer's disease or age-related mental decline? Quite possibly. In recent years, circumstantial evidence has been building that training the brain with stimulating activities can help compensate for the physical damage that occurs with age. This, in turn, may allow people to avoid symptoms of Alzheimer's or mild cognitive impairment far longer than others whose brains aren't so adaptable. Columbia University neuropsychologist Yaakov Stern, an early champion of the theory that you can build up a "cognitive reserve," compares the aging brain to a telephone network with some of its lines knocked down. "If you have a big enough network and Yonkers gets knocked out, you can still route the call through New Rochelle," he says.