Critical thinking emerges today as a fundamental, indispensable tool for every single citizen, since it allows each one of us to deal with the overflow of knowledge, the problem solving and the decision making in an enlightened, efficient, capable way. Therefore, critical thinking must have a prominent place when we speak of teaching in general and, particularly, of science teaching as science requires the use of skills such as: to analyse procedures and results, to use information properly and to solve problems. On the other hand, if we take into account the current state of knowledge, it is undeniable the role of social interaction, namely of peer interaction, as a facilitator of the critical thinking development (Azimitia, 1988; Cohen, 1994; Johnson & Johnson, 1999; Santos, 2000; Teixeira, 2001; Teixeira & Oliveira, 2002, 2003). Vygotsky (1978) defends that the most complex cognitive skills originate and expand in the interaction of each individuals with others. In fact, when peer interaction is used as a teaching-learning strategy, the pupils are induced to interact with each other, contributing for the construction of their own knowledge and for the development of critical thinking. However, it is not enough that students sit side by side for promoting a peer interaction enhancing critical thinking. Also, we know from previous research that altering the "didactic contract", defined by Schubauer-Leoni (1986) as the rules that condition the didactic relationship among the different elements of a classroom is enough for pupils to feel stimulated to interact effectively. So, we tried to identify, through exploring and testing, other factors that might facilitate peer interaction. For that purpose, and during a whole school year, a study was carried out in three Portuguese schools, which involved 5 teachers and 144 pupils who were attending the 5th and 6th grades of the Basic Education Program. The data were gathered by observation, by analysising the audio transcripts of pupils' peer interaction and pupils' written statements. The results of the study point out that the efficiency of peer interaction as a promoter of critical thinking in science teaching and as a facilitator of scientific conceptual development is influenced by the association of several factors, namely: - the use of a written "pattern", explaining the rules of the new "didactic contract" and always present in each of the pupils' activities, that was deliberately conceived in order to provoke peer interaction; - the choice of a kind of pupils' activities that due to their own nature, appeal to peer interaction, as for example, activities involving discussion, problem solving or decision making; - a way of writing the items of the activities that compell to peer work. For example: "Try to find out your peer's arguments". - strategies that invite to peer interaction. For instance, by asking one of the pupils to play the tutor's role with his peer: "Explain to your peer what you have studied", or "Discuss your ideas with your peer". During the presentation other examples will be given and we will explore some curricular proposals that may facilitate peer interaction and that will also, inevitably, appeal to critical thinking.