This work draws attention to the question of how in-service teachers learn to teach computational thinking and programming across subjects in K-9 education. Drawing on qualitative analyses of 298 reflective notes provided by 120 in-service teachers attending a professional development program, we pay attention to the following emergent themes: i) developing an understanding of the subject of computational thinking in K-9, ii) connecting programming with the teachers’ own subject matters, iii) understanding the purpose of teaching computational thinking. These themes point at the importance of scaffolding in-service teachers to learn to program with educational materials, tools and programming environments but also with a sound and inspirational pedagogy that addresses the fundamental questions of: what, how and why programming and computational thinking should be part of the compulsory school curriculum?