The principles discussed in the last chapter are sufficient for teaching words that are labels for concepts that children already know, or at least that can be explained in terms of experiences that are already familiar-that is, words that have easier synonyms or can be easily defined. However, often a teacher needs to teach complex concepts, such as DNA, independence, quadratic equation, and so on. What makes something a complex concept depends not only on the word, but on the background knowledge and ex periences of the students, and on how thoroughly they need to learn it. In general, what makes something a complex and difficult concept is not just that the idea itself is new to the child, but also that it is part of a new set or system of ideas. Learning the meaning of the word ventricle, for example, depends not just on mastering one concept, but also on understanding how the different chambers of the heart are related to each other.