Science tries to explain a very complicated world. We are surrounded by very many objects, moving around, reacting together, breaking up, joining together, growing and shrinking. And there are many invisible things, too - radio waves, sound, ionizing radiation. If we are to make any sense of all this, we need to simplify it. We use models, in everyday life and in science, as a method of simplifying and making sense of everything we observe. A model is a way of explaining something difficult in terms of something more familiar. For example, there are many models used to describe...