Getting the right answers in maths is only half the problem. Understanding why what you're doing works is the part that often stumps students and teachers alike. The essential guide for mathematics teachers and those training to teach, "Yes, but Why?" answers all your questions, and sheds light on the hidden connections between everything in mathematics at school. This second edition includes: (1) A new 'Test yourself' feature in every chapter; (2) More coverage of the four operations; (3) Enhanced discussion of fractions and proportionality; and (4) Downloadable figures for use in the classroom.