This research is a research and development which produces teaching materials. The research method consists of stages, namely: an introduction (literature study, observation, and interviews), product development of teaching materials, test by a team of experts, and restricted trial to the eleventh-grade students of senior high school in Cimahi, West Java, Indonesia. It was concluded that the teaching materials through scientific approach with what if not strategy load some activities including observing, trying, reasoning, asking, and concluding. The activities of asking using what if not strategy involve changing the data, adding the data, changing the data with the same question, and changing the question to the same data. In the teaching materials, it is presented some contextual problems in accordance with the experience of students. The design of teaching materials consists of three parts, namely: the beginning involves cover, preface, and table of contents; the content section: containing the title of the chapter dealing with the enumeration rules as a representation of the contents of the chapter; and closing: as a reflection and evaluation of the material presented on teaching materials for the next revision. Teaching materials can enhance the ability of reflective thinking of the students with medium criteria.