Use it or lose it. It's a cute saying, but it is particularly fitting for the brain-especially in small, developing brains. "During the first three years of life, there's an overabundance of activity in the brain," says Kenneth A. Wesson, education consultant at Neuroscience in San Jose. "If brain cells don't find a job, they will be eliminated. There is no welfare in the brain. These brain cells seek a job to do. They go dormant or are eliminated if kids don't have specific kinds of experiences and nothing to build on."