changes over time as conditions inside the body and in the external environment change, With these changes, behavior changes. For example, late in the day (or night) most people get tired and slow down. As the time since you last ate increases, you are more likely to feel hungry. When an interesting person walks in the room, you may stop what you are doing and go up to meet her/him. Motivation is used to explain how and why such changes in behavior occur. Motivation is very closely linked to reinforcement. A high level of motivation for something (food, social power, cigarettes, etc.) will make it easy to use it as a reinforcer. ]