Intro
Your brain is a thinking organ that learns and grows by interacting with the world through perception and action. Mental stimulation improves brain function and actually protects against cognitive decline, as does physical exercise.

The human brain is able to continually adapt and rewire itself. Even in old age, it can grow new neurons. Severe mental decline is usually caused by disease, whereas most age-related losses in memory or motor skills simply result from inactivity and a lack of mental exercise and stimulation. In other words, use it or lose it.

Only recently have scientists been able to learn how the neural network of the brain forms. Beginning in the womb and throughout life this vast network continues to expand, adapt, and learn.

Step back a half-billion years ago, to when the first nerve cells developed. The original need for a nervous system was to coordinate movement, so an organism could go find food, instead of waiting for the food to come to it. Jellyfish and sea anemone, the first animals to create nerve cells, had a tremendous advantage over the sponges that waited brainlessly for dinner to arrive.
After millions of generations of experimentation, nervous systems evolved some amazing ways of going out to eat. But behind all the myriad forms of life today, the primary directive remains. Movement. In fact, a diminished ability to move is a good measure of aging. Inflexibility heralds death, while a flexible body and fluid mind are the hallmarks of youth.