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 preservice 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.