This paper reports research into the effect on 11-year-old pupils of introducing more cognitively challenging, practical, and interactive science lessons. Our hypothesis was that such lessons would increase the children's enthusiasm for science and their engagement with the scientific process, thereby improving educational performance. Schools in England are under pressure to raise achievement, as measured by the results of national tests. This has an impact on teaching, where revision of subject knowledge often dominates and can be particularly detrimental to more able pupils. The research was a controlled trial which took place in thirty-two English primary schools as part of a project "Conceptual Challenge in Primary Science". Teachers from 16 intervention schools participated in continuing professional development (CPD) and developed science lessons that had more practical work, more discussion, more thinking and less (but more focused) writing. The proportion of pupils achieving the highest level (level 5) in the national science tests at age 11 was compared in the matched-school pairs before and after the intervention. Focus group interviews were also held with a group of pupils in each intervention school. There was a 10% (95% Confidence Interval 2-17%) increase in the proportion of children achieving the top score in the intervention schools. The pupils and teachers reported greater engagement and motivation. These findings suggest that moving from rote revision to cognitively challenging, interactive science could help improve science education. They merit replication in other international settings to test their generalisability. (Contains 2 tables.) [Funding for this project was provided by the AstraZeneca Science Teaching Trust.]