The aim of physics education is to generate understanding of physics and there is considerable anecdotal evidence that passing examinations in physics is not the same as understanding the subject. This paper describes how the areas of difficulties in understanding of physics were determined for pupils in Taiwan aged 13-15. Test material which placed little load on working memory was then developed for several of these areas and the pupil performance was related to measured working memory capacity. Those with higher working memory capacities were found consistently to understand the ideas of physics better. The implications are discussed. (Contains 7 tables and 1 figure.)