highlights that the abstract physics energy concept is a quantity of state of a system. Therefore, energy can be viewed as being 'stored' in a system. But the changes of energy numbers in interacting systems also allow a view of energy as being"exchanged' between systems. In most practical uses, the exchang aspect is the more important one. Of course it matters how much energy is stored in a system but how much energy one may get out of a system depends on the changes the system undergoes (c.f. Carr and Kirkwood, 1988). Very often the exchange aspect is conceptualized by employing the idea of energy flow. This idea has become popular in science teaching. Energy is viewed as something that flows from one system to the other if there are interactions that result in the change of the energy numbers in the two interacting systems (as indicated in figure 14.1). This idea is helpful in easing access to the abstract energy concept because it provides a somewhat pictorial underpinning of the abstract change-of numbers idea. The idea of energy conservation is very obvious within this conceptualization if the energy flow is viewed as the flow of an indestructible entity. Whereas energy transport is only a change of numbers, in systems in the above abstract conceptualization it may be viewed here quite pictorially as the flow of the entity energy.