This article outlines a model for thinking about the effectiveness of practical activities in school science and how this might be evaluated. This was used in a research study of current practice in the use of whole-class practical work in secondary schools in England. The emphasis in the lessons observed was on successfully 'producing the phenomenon'. Little whole-class time was used to discuss the ideas that the activity involved. Task design did not reflect the wide variation in task demand. This suggests a need for greater clarity about the learning objectives of practical activities, and wider use of strategies to increase the 'minds on' aspects of practical work.