What is it that determines how effectively students work on a task in the open phases of a school lesson? At the European Conference for Educational Research (ECER) in Budapest we presented our latest research results on this. Of course, a student’s own motivation can explain his or her effectiveness to a large extent. But what is surprising is the strong influence the learning partner has on the learning results. Also similarities between the learning partners have an impact on their effectiveness: The more similar the learning partners’ motivations are, the better they learn together.