People in India may not know much about baby boomers (those born between 1946 and 1960) in the US, but this generation has provided the majority of the country's leaders in politics, science, business and the arts. They have been credited with building several institutions, and in some ways, with building contemporary America itself. One of their less well-known achievements was to start the physical fitness revolution. Now they are on the verge of starting a similar revolution: one of mental fitness. As biology advanced rapidly after World War II, scientists began to understand the relationship between good living and major diseases such as cancer, diabetes and heart ailments. Many baby boomers took this knowledge to heart, thereby starting the physical fitness movement and an entirely new industry. Now science is generating a burst of knowledge about the brain. And based on this knowledge, Americans have begun to generate another industry: the brain fitness industry. It is already worth $225 million in the US and growing fast. Science now tells us that the brain is plastic, and remains so well into old age. It seems an obvious fact to state, but it is a counterintuitive concept and was not accepted by many neuroscientists till the 1980s. Even in the 1980s and 1990s, the real extent of the brain's plasticity was not widely appreciated. They thought that the brain was hardwired early in life, and that all its mass and circuitry were established in the first few years of life. Says Michael Merzenich, professor emeritus at the University of California in San Francisco and founder of Posit Science, a maker of brain fitness tools, "In spite of evidence, people did not believe that the brain can remain plastic in old age.' Merzenich himself was at the forefront of the transformation that changed our notions about the brain. Now it is known that the brain