One of the most effective methods of teaching children--and others--about complex processes and systems is by comparing them to simple and familiar concepts in their own lives. The bloodstream, for example, is often described as a train, transporting "goods" like oxygen and nutrients to the cells, each of which is like a station. This type of explanation enables the student to form a mental picture of the functions described, whether or not they fully understand the true mechanism at work. Thus, it is not necessary at a basic level to explain osmosis and selective membranes and the host of other complex processes involved in transferring oxygen and nutrients from the blood to the cells of the body; to say, "The blood 'train' carries these things to the 'stations,' the cells, and drops them off before heading back to the heart to pick up more." This is much easier to explain and much simpler to understand, and is a sufficient explanation for many purposes.
             Another process which includes very complex details, but which can be fairly accurately explained with broad strokes, is the hydrological cycle. The basic processes of the hydrological cycle are precipitation, evaporation, and condensation. The hydrologic cycle is a true cycle; there is no beginning or end, but the rather these processes are all in constant motion. This three-step model of precipitation, evaporation, and condensation is already a simplification of the process, even disregarding the molecular changes that occur to allow it to happen. In reality, each of these process is made up of and affected by other, smaller processes. The complexities of this system can make it difficult to teach, and difficult for students to understand, which is why coming up with a familiar and analogous explanation, like describing the bloodstream as a train, could be useful. For the hydrological cycle, because it is both cyclical and a mu...