The main aim of this project was to study young people’s ability to use science knowledge when talking about and explaining everyday phenomena involving transformations of matter. Students’ individual knowledge was studied both as their spontaneous explanations and as their explanations with appropriate help in discussions with the researcher or with other students. The framework for learning in this study was a social constructivist perspective of learning. In the project students discussed everyday phenomena with peers and with the researcher. The role of the discourse was stressed in the interviews as well as the development of students’ use of their mental models when explaining everyday phenomena. Data were gathered through four interviews with each one of 40 students, between 9 and 11 years of age.. The development of the basic particle model was one of the themes during the instructional units. Other recurrent themes were states of matter, gases and chemical reaction. Most of the students were able to use knowledge of science when talking about known everyday phenomena. Almost all of the students in the group developed the use of their own mental models during the project.