This article introduces a project in a Finnish secondary school where mathematics education was combined with crafts instruction. The idea was to provide the students with an interdisciplinary real-world learning context in which they worked collaboratively on an open-ended design task. The approach was problem-based and student-centred in a way that Neumann (2013) describes to be in and with students. The teacher's role was to allow the students to self-generate their learning and to work in partnership with them. The students were given a rather open-ended, ill-defined design task that required them to take risks, find information and collaborate. The student interviews (N = 17) after the project revealed that their attitude to mathematics had become more positive. They began to understand the connections between these school subjects in an authentic learning environment. They learned to solve problems and combine theoretical and practical knowledge. Their understanding of the importance of mathematics in real-world situations increased. The project demonstrates how to arrange teaching and learning in a more holistic way instead of in a traditional subject-based approach. This kind of interdisciplinary approach demands good cooperation from both the students and the teachers. For interdisciplinary education to be further developed, support is needed for the teachers to collaborate and learn new teaching approaches.