Hull’s Model states that a body wants to stay in a homeostatic equilibrium.  If a part of the body’s system is thrown off balance, the body will work to go back to a level state.  Hull's Drive-Reduction Model implies that behaviors are caused by a corresponding lacking in the internal environment.
     This can be confusing so here is an example.  When you get home from school, you are hungry.  Because the body is off balance you ask mom when dinner will be ready and you sneak a snack to ward off the hunger.  The body’s off balance stimulates the behavior necessary to get food.  Likewise, after the huge meal, you don’t feel like moving, you sit down and watch TV.  Once again, the body is thrown off by too much food and stimulates the behavior to rest and let it digest.
     However, Hull’s model has been criticized because it fails to explain why humans have the drive to do certain actions, like sports, surfing, juggling, etc.