Many disease processes act directly or indirectly on the central nervous system to cause an alteration in the level of consciousness. Although in severe cases the child appears unresponsive, the initial changes in mental status are often subtle. These changes are recognized by assessing the child's actions and responses to external and internal stimuli in what is collectively known as behavior. Behavior is a combination of a variety of observations, including appearance, level of alertness, speech, affect, mood, thought, and judgment. These actions are child and age specific. For example, apprehension, avoidance, and crankiness may be normal responses of a toddler to a physician's physical examination, whereas a similar response by an adolescent may be abnormal. Although major changes in behavior are readily apparent, subtle changes are often best appreciated by parents and caretakers.