Background: Understanding technological systems is essential for students to participate in and critically engage with a technology-rich society. Prior research indicates that pupils often find it challenging to grasp the structure, function and dynamics of such systems. Purpose: This study investigates how Year 6 pupils (aged 10–12) describe, explain and analyse a technological system–specifically the wastewater system–and what their self-drawn models reveal about their systems thinking. The aim is to contribute knowledge on how pupils conceptualise complex systems and how this can be pedagogically supported. Sample: Seven pupils from different schools in Sweden participated in individual interviews. All had received instruction about the wastewater system, including a study visit to a treatment plant. Design and methods: Data was collected through semi-structured interviews. Pupils were asked to explain the wastewater system and draw a model of it on paper. Thematic analysis was conducted inductively to identify patterns in the pupils’ verbal and visual descriptions. In the discussion, the Freiburg model of systems thinking was used as a conceptual framework to interpret the findings. Results: Pupils often began their reasoning from system-related problems, such as blockages. Most described the system as consisting of subsystems and components, although some expressed a more linear or circular view. Their self-drawn models supported verbal explanations but also revealed misconceptions about flow mechanisms–especially the role of gravity, which was often overlooked or replaced by flushing or pumps. Conclusion: The study shows that pupils’ drawings help structure their reasoning and support systems thinking. However, simplifications in the models may lead to misunderstandings. Teachers should guide pupils through technological systems ‘from source to end’, clarify key concepts and scaffold the model-building process to enhance accuracy and holistic understanding.