It is very cold in the Stone Age, but Pod has a brilliant idea how to keep his bottom warm... underpants! But he can't seem to find the right material! Written by Rebecca Lisle, this educational Early Reader book is book band Purple (aimed at 6-7 years, Year 2).The colour grading system is recognised by the Institute of Education which reflects the reading difficulty. The book content, language style, phonics and layout are taken into account to determine this.