The new computer science curriculum in Czechia for grades 6 to 9 accentuates, among others, the topic of computer principles, i.e., how computers work internally. For various reasons, there is a lack of teaching materials in Czechia that deal with this topic and support teachers in their teaching. To create quality materials, as a first step, we wanted to discover children’s preconceptions about computer principles. To this end, we conducted semi-structured interviews with 53 children about how computers work, what parts they comprise of, and how these parts communicate and work with each other. After completing an inductive thematic analysis, we found around 260 preconceptions. Based on their occurrence we performed a non-hierarchical cluster analysis and identified three groups of participants according to their level of knowledge (i.e., experts, who are able to understand the majority of abstract concepts; moderate, who understand some abstract concepts; and naïve, who do not understand the vast majority of abstract concepts). This poster presents preliminary findings from our study.