A model organizes what we think we know about something in order to predict how it might behave in the present, future, or past as well as how it might respond to external influence. Models are especially useful when direct, controlled experiments are difficult or impossible. A model can be a simple concept, for instance: the heart is a circulating pump. A possible prediction from this model is that blood in the body regularly moves through the heart. Or a model can be a series of mathematical equations that are solved by computers. This second type of model can be built upon the first model by applying the physics of pumps to calculate how much blood cycles through the heart in a minute.