Twelve lower-secondary school teachers in Mathematics and Science were asked to teach a topic of their choice during a lesson that was video-recorded. We were able to analyse 10 of the cases and we found that all of them were similar in one respect: concepts and principles were introduced one at a time, each one followed by examples of the concept or principle in question, apparently to highlight its essential meaning. All the teachers participated in 3 modified lesson studies with 3 cycles in 4 different groups during a year. The modified lesson studies built on a theoretical idea supported by a large number of recent studies. The theory states that new meanings (of concepts and principles, for instance) cannot be appropriated by engaging with instances of the same concept or principle, but through engaging with contrasting instances of contrasting concepts and principles. It is assumed that meaning derives more from differences, than from sameness. After the year of modified lesson studies, the teachers were asked to teach the same topic they had chosen the first time. The lessons were recorded again and the analysis showed that there was one thing in common in all cases: all of the 10 teachers dealt with the relevant concepts and principles in relation to each other (i.e. simultaneously) and not one at a time. By thus bringing out the differences between them, their meaning was made possible to grasp for the students. The study lends support to the conjecture that the modified lesson study is a powerful tool for enabling the teachers to structure the content of their teaching in accordance with a principle that is more powerful in making learning possible, even if this contradicts their taken-for-granted practice. © 2016 Taylor & Francis