The forebrain is the largest and most highly developed part of the human brain; it consists primarily of the cerebrum and the structures hidden beneath it. The cerebrum sits at the outermost part of the brain and is the source of intellectual activities. It holds memories, allows you to plan, enables you to imagine and think, to recognize friends, read books, and play games. The cerebrum is split into two halves (hemispheres) by a deep fissure. Despite the split, the two cerebral hemispheres communicate with each other through a thick tract of nerve fibers that lies at the base of this fissure. Although the two hemispheres seem to be mirror images of each other, they are different. For instance, the ability to form words seems to lie primarily in the left hemisphere, while the right hemisphere seems to control many abstract reasoning skills.