A smart brain is an adaptable brain; it processes and accommodates new information on a continuous basis.
The brain evolved through our evolutionary development as an information processor, bringing the “outside inside” so that the whole organism is privy to environmental stimuli. Primitive brains react only reflexively. As we move up the evolutionary ladder, the higher vertebrate brain emerged because natural selection favors brains that respond rapidly yet are flexible enough to adapt to changing environments. Although this is an understanding of the “smartest” brain based on the evolution of animals, it is, too, a good understanding of human intelligence. The best human brain is one that processes the widest range of stimuli, and adapts to changing circumstances the fastest. All learning is, in fact, change. Learning takes place only when a new stimulus has presented itself to the brain. As circumstances change -- and these may be external circumstances (like reading a book), or internal circumstances (like moods) -- the brain changes to accommodate the new set of information it is receiving.
A smart brain is one that can accommodate change; it can build new internal connections, grow new tissue, and gain new capacities and speed. This means that we can always keep learning, not only by taking in new information, but also by improving how we process that information. The implications of this discovery are fundamental and profound.