These brain-based abilities and traits are constantly affected by both environmental and physiological factors from childhood through old age. For example, the brain is constantly changing, as evidenced by the hierarchal development of cognitive skills in children (for example, concrete to abstract reasoning, the development of reading and academic abilities, or learning how to socialize with other children) to the decline of cognitive (such as forgetfulness) and physical abilities (such as vision, hearing, and motor skills) in old age. Furthermore, the brain is made of millions of neurons and glial cells in complicated neural networks that still are not fully understood. Injuries to any of these neurons or networks can affect a multitude of skills. Neuropsychological abilities (that is, cognitive, behavioral, and emotional abilities) are controlled by many factors, including brain structures (such as cortical tissue, blood vessels, and protective membranes), electrical impulses, neurotransmitters and chemicals, protective and nourishing uids (such as cerebral spinal uid or blood), and other homeostatic functions that regulate and maintain specic brain abilities (for example, intracranial pressure). Obviously, the brain is a very complicated organ and its many functions can be affected by many factors.