The brain is the most complex organ in the body. It consists of more than 100 billion cells. (Compare that with the human population, which is only 7 billion.) Unlike the cells of any other organ, brain cells interact extensively with each other every millisecond. The brain's nature as a dynamic, massively interconnected network is the basis for its ability to process information. It is also the basis for learning, memory, and plasticity. These latter properties are formalized as changes in brain function, and they take place throughout our lives. These changes are cumulative, as illustrated by how our education, memories, habits, and injuries build on each other, whether we like it or not. Our understanding of the brain's cumulative nature forms the basis for all sorts of interventions, giving us hope that they will have a lasting effect.