The big ideas of science education are the current changes in the science curriculum all over the world and also in Chile. Experiences of lessons using big ideas are, mostly, from the Academia describing what and how teachers teach in their classrooms, generating a lack of the teacher´s practice since their own voice. This research explores, under the collaborative self-study methodology (Bullock & Russell, 2012), the practice of an undergraduate level´s teachers where it is developing the reflexion and self-knowledge about the Big Idea of living being. The results show that it is possible to build with the students a Big idea from “small” ideas since the analysis of concrete examples. Furthermore, for this teacher, the big idea as a guideline that promotes more flexibility and confidence in the students´ knowledge. Finally, considering critical friendship and the sharing of the experience benefit reflexion of the practice and learning of everyone.