This Unit looks at how mathematical ideas may be effectively developed in the classroom through discussion, writing and practical work. The topic of ‘shape’ is taken merely to illustrate the arguments presented. It considers how ‘mathematical’ words can either clarify or confuse. Unit 1 suggests ways of encouraging talk in mathematics and this Unit builds on some of these ideas. Other Units, such as 3 and 4, provide opportunities for ideas to be developed and represented in problem-solving situations.