Thanks to brain-imaging studies in humans and neurochemical studies in animals, scientists have found that exercise actually develops a stronger brain. Physical exertion induces the cells in the brain to reinforce old connections between nerve cells and to forge new connections. This denser neuron network makes you better able to process and store information, essentially resulting in a smarter brain. A substance called BDNF, for brain-derived neurotrophic factor, helps nerve cells grow and connect, speed up healing of brain injuries and warding off depression. It works by actually rewiring the brain. BDNF if one of the tools a brain uses to turn life experiences into long-lasting changes, influencing everything from memory to mood.

Exercise does more than simply preserve brain tissue -- it can also improve thinking. In a recent comparison of 18 studies, inactive older adults who began an exercise routine got significantly better at cognitive tests than measured skills such as planning and paying attention. The kind of exercise that makes a difference is not hard to tackle. A brisk 45-minute jaunt three times a week will do it. Even young people can expect to see some brain benefits from exercising -- an improved ability to handle complex cognitive tasks and protection against the effects of aging.