The human brain is divided into three main parts: the cerebrum, the cerebellum, and the brain stem. The cerebrum is the largest part of the brain (about 85 percent of its total weight). It controls emotions, thought, memory, and speech. It is divided into a right and left side, called hemispheres, and each side is divided further into parts called lobes. Its thick outer covering, called the cortex, is made up of a type of tissue called gray matter. The cerebellum coordinates the kinds of movements we don’t usually think about: it helps us walk upright and in a straight line, it keeps us balanced so we don’t tip over, and it gives us coordination so we can run and play. The brain stem connects the brain with the spinal cord. It controls our body’s vital processes, like breathing, digestion, and heart rate.