Learning by teaching is an educational method developed by Jean-Pol Martin for foreign language lessons and is commonly used in Germany. This study discusses a reconstruction of learning by teaching as developed for use in a science context where in the adapted version of this method is introduced, the views of pre-service science teachers are explored, and the effectiveness of this method for gaining 21st century skills is examined. The qualitative case study was conducted with 33 pre-service science teachers. Data was gathered through focus group discussions and structured written interviews, and analyzed using content analysis. The views of pre-service science teachers were examined concerning the advantages of the following 11 themes: the advantages of learning by teaching, preparing research questions and working together to find answers through research, preparing and implementing activities, the role of the teacher, a letter writing activity, the strengths of learning by teaching, the weaknesses of learning by teaching, difficulties faced during its implementation, the use of learning by teaching in teacher-training programs, other lessons appropriate for the use of learning by teaching, and how to apply the experiences gained to the future. Following a two-year pilot study and implementation of the final version, learning by teaching within the context of science was found to be an effective method for gaining 21st century skills