The teaching of words, and of the meanings and concepts they designate or convey, is one of the principal tasks of teachers at all levels of education. It is a concern of textbook writers and programmers of self-instructional materials as well. Students must be taught the meanings of unfamiliar words and idioms; they must be helped in recognizing unfamiliar ways in which familiar words may be used; and they must be made generally aware of the possibility of ambiguity in meaning and the role of context in resolving it. Often the task that presents itself to the teacher is not merely to explain a new word in familiar terms, but to shape an entirely new concept in the mind of the student.