Our basic question is how do school-age children think about variables and mechanisms in areas that are relevant to physics. We wish to know which variables and mechanisms they recognise, and how they organise these variables and mechanisms in their knowledge structures. In part II we asked the question in relation to the transfer of heat. We found that children recognise a number of variables, many of which are consistent with received science orthodoxy. However, while this was true at all levels of schooling, there was evidence of enhanced understanding with age and in particular of substantial progress around or after 11 or 12. A similar picture emerged with mechanisms. The majority of school-age children appreciate that heat is transferred, but recognition of the transmissive nature of transfer is rare before the teenage years. When we shifted attention to knowledge organisation, it became apparent that the parallel age trends for variables and mechanisms were probably not accidental. There was a close association between progress over variables and progress over mechanisms which was suggestive of contingency. In particular there were signs that progress over variables is dependent upon progress over mechanisms, indicating a generative role for the latter in relation to the former.