It is interesting to note how math in textbook is likened to junk food. This is read in an early report in England explaining what's wrong with common practice in teaching to low attainers in math. I believe the advice is still valuable for classroom teaching today and in HK.
The report, Better Mathematics, published by H.M.S.O. in 1987, is now available in the National STEM Centre eLibrary [http://stem.org.uk/rx7ca]. The comparison above is extracted from p.15 of the report. That section describes the poor "mathematical diet for low attaining pupils". Nowadays, our textbooks and all sorts of ready made teaching resources look even more attractive, and can easily give an impression that you can have a lot to 'eat', but in fact nothing good for your health.