Sometimes, what must be understood is demanding for the learner. It may, for example, take too much of the available mental resource to process, it may be too abstract or it may call for other knowledge and understanding which, at present, is beyond the reach of the learner. In short, the learner fails to build a working model – an internal representation – of what is to be understood in his or her head. But this does not mean understanding is impossible. We can often help learners by giving them something to think with. This something – an external representation – captures the essence of what is to be understood. It could, for instance, be something as simple as a gesture or movement or it could be a picture, a diagram or a chart. For example, very young children can interpret and use simple gestures to communicate and grasp thoughts (TrepanierStreet, 2000). The human body can also be used as an aid to understanding with older children, as when we help them grasp the short amount of time that Homo sapiens has existed using someone with an outstretched arm. If the distance from the nose to the finger tip stands for one billion years, it would take more than four such people to show the age of the Earth but only a thin shaving of skin on the finger tip of one of them represents the duration of our existence (Law, 2003). Such representations are devices which enable the learner to see relationships more readily and make reasoning easier. The graphic organizer is an example of such devices.